The biggest disadvantage I see comes from DIR's emphasis on the dive team. Paradoxically, I also see this is as one of it's greatest strengths.
The idea of training and diving with fellow DIR divers who all have a standardized equipment configuration, consistant methods, and a common understanding makes tremendous sense in technical applications like deep or penetration diving, and the fact that DIR has allowed such dives to be accomplished with a respectable safety record speaks for itself. Certainly, DIR can be applied to even the simplest of recreational dives, and will make those dives run more smoothly and enjoyably.
Personally, I don't believe it's necessary for someone to be DIR in order to be a good dive buddy or a safe diver, but diving with buddies who are not DIR is not DIR diving.
I have many diver friends who've never taken DIR-F, and have no intention of doing so. That I can not have safe and enjoyable dives with them and and still consider myself a DIR diver is the biggest reason I haven't fully embraced the philosophy. Given the choice of either diving DIR or adopting many of the principles of DIR and continuing to dive with non-DIR friends whose equipment and styles I'm nonetheless familiar and comfortable with, I'll take the friends and toss the title.
I think what I've learned from DIR-F is very valuable, and I'm continuing to drill the skills and techniques I learned there, but I don't consider myself DIR, and am always happy to get out and dive with anyone whom I consider to be a reliable buddy wether or not they've tasted the kool-aid.