I have to say that I am solidly on the side of seeing (good) airsoft guns as a valuable addition to self-defense training.
I own several airsoft guns which replicate real steel (Glock, 1911, AR-15, etc.). The handguns are all gas blowback -- so other than not having the muzzle blast and heavy recoil, they function exactly like the real thing. Being nearly identical in size/dimention, I can use my real steel holsters and pouches, which is nice.
You can practice reloads, malfunction drills, draws, sight alignment/sight picture, trigger control, muzzle awareness, etc., etc., from the comfort of your own home with little risk of doing more than cracking a picture frame with a stray airsoft pellet (I wont tell you how I know that).
I've found them to be particularly useful in helping to train for various scenarios within the house. I can take small or medium sized shipping boxes and fill them with anything soft (crumpled newspaper, packing peanuts, a pillow) and place them at various locations throughout the house and actually shoot into them safely. That setup effectively captures the pellets so you dont go dinging your walls or breaking glass anywhere (unless you miss, that is).
I know a lot of people knock the value of airsoft guns -- but if you get a high quality one (prolly $150+ for a handgun, or $200-500 for a long gun), they are extremely realistic and most will function exactly like the real thing. I think where people get disappointed is when they buy the cheap guns which dont operate like the real ones.
Good luck!