The reason I'm writing this post is because it seems to me that we should get rid of Professional REL as a separate concept, and move it into Comp. REL.
The first justification I have for this change is that Professional REL just complicates the system of competitive magic. Apart from describing the philosophy of Prof. REL, the one time Professional is mentioned in policy documents on it's own is to denote that spectators can't stop matches for a judge at professional (and given the rarity of events run at Professional REL, adding this as a requirement for running these events seems simpler).
Second, the philosophy of Professional REL is very similar to Competitive REL. The description of Professional in the IPG only says that they:

Hold players to a higher standard of behavior and technically correct play than Competitive events.

Whilst this may be philosophically true, the infractions do not change, and downgrades will never be issued at Competitive where they wouldn't be issued at Professional (to my knowledge at least). Players at a PT already know that their competition and play must be excellent, but that's not a product of policy, but of the skill of other players. The skill of the opponents will generally also dictate that a firm grasp of the rules will be necessary, for reasons that should be obvious.
In addition, there are situations in which Comp. REL tournaments (or a part of a tournament that is at Comp. REL) are, according to philosophy, being run at Prof. REL. The most common example of this is coverage, in which people are physically unable to stop a game in order to call a judge, and players are acutely aware of the fact that they are facing another very good player and are under scrutiny from anywhere up to 10,000 people. There is also the fact that when players are low in the standings, there is also a somewhat lower standard of play (though not judging), as prizes are then either impossible to attain, or impossible to improve given their standing.
Thus, I can't see a reason why Professional REL needs to exist, and it would seem that removing it makes the IPG a little simpler, and reflects the natural fact that there is a spectrum which isn't clearly delineated by a the day of an event, or the type of that event.

I think you might be under valuing the effect of hearing “today will be run at Professional REL” might have on a player who made day 2 of a GP. While large number of players at a Pro Tour are use there are also those who have won their first Ptq and aren't used to what is expected, knowing that the event is Professional REL will make the experience feel like more of an accomplishment for those players.

TL;dr while for many judges and some players it makes no difference for a majority at a GP day two and a small percentage of a PT field it does

I, having never reached a PT or day 2 of a GP, may certainly be underestimating that (though, as a judge who knows what Prof. REL means, I would be biased towards your point of view), but I think that hearing “today will be run at Prof. REL” isn't as important to a non-judge as the actual making of day two, or facing another amazing player, or something else like that. I do think players will know the rules, but i don't think that particular statement of it being at Prof REL. is much more impactful than any of the other pertinent event info. I personally value the efficiency and philosophy of the document over making a people at the PT feel (marginally) better.

There is a very significant difference between Professional and Competitive REL, defined in policy; spectators may not pause a match if they think they see an infraction.

The quote you included is also significant. “but I didn't know!” isn't nearly as convincing when you're expected, even required, to know…

We have discussed Pro REL, and so far we've concluded that we should keep it. Also, having it separate gives us “design space” within policy, in case we determine we need to implement other differences.

d:^D

Scott, I always wondered why we don't let spectators interrupt a game during Professional or the other way around: why do we let them interrupt during competitive?

Sorry to hijack the thread, but it the questions sprung to mind when reading your post.

There is a very nice explanation from Uncle Scott in this thread why we don't allow specators to stop a match at Professional REL:

Originally posted by Scott Marshal:

Considering the higher stakes, we really want to protect the integrity of the event. Spectators, Players, Judges - which of those do you think would be (a) most disruptive, and (b) least reliable, when it comes to spotting problems and acting on them?
At Pro REL, we have higher expectations of players - I'm not going to pretend that everyone who Q's for the Pro Tour, or makes Day 2 at a GP can meet those expectations, but that's the stated expectation. It's up to them to meet that requirement - and most do.
As such, we have to trust players - and the much higher-than-normal concentration of judges - to act as our first line of defense against misplays and even shenanigans. Not spectators - who, as it happens, are rarely unbiased, nor as attentive to each detail of the current game. Even the best & brightest among Players may take an action that, unintentionally and unbeknownst to them at the time, undermines their actual goal of protecting the integrity of the game.

Edit: better explanation why quoting the thread

Edited Auzmyn Oberweger (Dec. 19, 2014 10:19:12 AM)

Scott, I think the idea of keeping extra design space is fair enough, and since you say you've discussed Professional REL I can be sure that a due amount of thought has gone in to it. A slight philosophical question, however: when does “I didn't know” ever effect Competitive REL? No infraction I could find would be downgraded due to a lack of knowledge, so where would that ever make a Difference?

Edited Tom Kunc (Dec. 19, 2014 04:04:45 PM)

Me: “you know, you can't ignore Lifelink, it's not a Trigger…”
Player: “what?!? no … I didn't know that…”
Me: (believes player) “OK, this is a Warning, for a Game Rule Violation …”
-or-
Me: (doesn't believe they didn't know) “OK, I'm going to disqualify you from this event …”

The threshold for believing that a player does or doesn't know a fairly common rule or game mechanic - e.g., Lifelink - varies between a PTQ, or day 1 of a GP, versus day 2 of that GP, a Pro Tour, etc. For that matter, our tolerance of what a player should know often varies by REL.

d:^D

“I didn't know” is one of the important factors when investigating a DQ.

If a player did not know what they were doing was wrong they do not get DQ'd. That applies to most cheating but not Rando Det a winner, bribery, collusion.

Ah, I didn't consider the diference between cheating and any other infraction. Thanks Scott and Nick.