matt_unique:
You will soon discover the number 1 reason to avoid DIR as your only resource for improving diving skills; it's the attitude of many (not all) DIR divers. This attitude is indeed "there is only one way to dive, there is only one acceptable configuration, and you just don't understand DIR if you do not agreed with our methods". Ask any diver who is not DIR and they will likely share this impression.
I'm still waiting to meet the "many" DIR divers who share this attitude. Of the couple dozen people I know around here (in real life, not on the internet), I can think of one or two who can have a bad attitude about it.
As far as the original posters question goes, DIR can be great for divers who start out since it is a complete system that doesn't have a lot of boundaries. A lot of divers who start out buying Air2's, jacket BCs and split fins buy gear all over again once or twice as they progress to more demanding diving. I don't know of a lot of people who have started with BP/W, long hose and jefins and gone back the other way. Its unlikely to be a choice that you'll later seriously regret. You might wind up with some kind of crazy sidemount gear, but you'll have to reconfigure your gear for that no matter how you start out.
OTOH, particularly for recreational diving, there's a lot of other choices. I'd suggest against jacket BCs and in favor of back-inflates, I'd suggest a long hose setup, but I dive with guys who use Air-2s. I'd suggest against pony bottles and in favor of team diving, but I've dove with guys who use 'em (but the more they look like a slung stage bottle, the better, imo). I'm sure there's probably good instructors out there, but I can recommend that in a DIRF class you'll get an enormous level of feedback with video review that I guarantee will turn you into a better diver. OTOH, I've dove with divers who were (IIRC) IANTD trained divers where I couldn't see any noticable difference between how they dove and how all the DIR divers I know dive, at least on recreational profiles.
I guess if you disagree fundamentally with any of the logic behind the gear choice, or the skills or the focus, that would be the biggest reason to not go DIR. If it all generally makes sense to you, there's really no reason not to go down that road though. You will have to put up with the occasional zealot. You may find yourself not entirely wanting to be so closely associated with a certain gear manufacturer (*cough*). You might not like the endless arguments over minutae. Mostly those are all pretty peripheral, though -- particularly for the recreational DIR diver.