Ugh, I hate that attitude. I had DMs that would pull this all the time - it doesn't matter how good my character might be at disarming traps, if I personally could not explain how I was disabling it - and somehow predict, despite not being a thief/rogue myself, how to bypass the dangers of the trap, it would go off in my face.
I only require it if I can't figure out how or why something would work.
The extreme example of your point of view is,
Thief: I steal his pants!
Dm: What? He's wearing them!
*clattering of dice*
Thief: Forty five!
Dm: Uh, I guess you steal his pants?
Party: Sweet...
So if you're disarming a trap with Thiever, I'm not going to quiz you. I figure you do thiefy stuff and maybe it works. But when you announce to me that you're disguising your male tiefling fighter as a halfling, I'm going to ask you how. Because it doesn't seem plausible on its face for anyone, no matter their skill level.
They shouldn't conflict with normal skill use, because they are taking things to another level. A character can still use basic disguise and forgery on their own, in the normal means - these just provide some additional methods that will generally be more effective for those who have invested the time and effort in mastering and using them.
That's a nice theory, but it doesn't really apply. The PHB doesn't mention anything about disguising yourself that I can remember. Certainly nothing in the Bluff entry. It would seem that the only official way to disguise yourself is with this training, or with magic. Likewise I can't seem to find any "regular" way to forge a document.
Those facts notwithstanding, I've been allowing this kind of thing with skill checks. I don't really feel that disallowing it and requiring a feat to accomplish the same thing is increasing value.
Like I said before, IF and WHEN these trainings provide new ways to use skills, I'll like them a lot. Its just not entirely clear that this has happened- instead, they retroactively take away an existing (although apparently houseruled) way to use skills, and then offer it back, slightly adjusted.
If I'm wrong, and the PHB offers a "regular" way to disguise yourself, and if this martial training stuff is different and better than the regular way, I'll happily take back everything I've said about this particular training if you just point out the page number or compendium reference where this is demonstrated.