I'd like to pretend this wasn't brought up by a certain well-known player's account of an incident at a large event this past weekend, but you all know better. With that said, I don't understand the reasoning behind not allowing a spectator to stop a match if there is a concern that requires a Judge's involvement. With the stakes being even higher at Professional REL, I would think it'd be even more important for a match to be stopped if there is a concern.

tl;dr Summary: What's the philosophy behind that sentence in the MTR?

Originally posted by Sean Stackhouse:

With the stakes being even higher at Professional REL

that's a very good reason to NOT have spectators involve themselves in a match, but instead to get a Judge involved ASAP.

I've been informed that my previous answer was far too succinct to make my point clear, so I'll try again. I'll admit, it was a bit of a rush job…

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.

I've also read a fairly common misunderstanding: Spectators are not just allowed, but encouraged, at Professional REL. The new layout of the Pro Tour floor that debuted in Valencia vastly increased the opportunities for spectators to watch any match from no more than about 3 meters distance. We want spectators - we just want them to spectate, not participate in matches.