According to the annotated IPG you need to analyze the whole card in a vacumn and not each ability separatedly in order to conclude if a trigger is detrimental or not.
The way I see it Bomat's trigger is clearly non-detrimental because the card would not be played without it AND it's an upside when analyzed in conjunction with its other abilities.
Can anyone confirm if this trigger is detrimental or not?

For reference: http://blogs.magicjudges.org/rules/mtg/standard-detrimental/

Why is Bomat Courier's ‘when ~this~ attacks’ trigger considered Detrimental? Because you're losing access to a resource. Yes, you might get that resource back later - but that's uncertain. What's certain about this trigger is that you're losing a card each time it attacks.

d:^D