I was a trainer for over a year at a local commercial gym and just recently quit to start my own gym with a friend. Having been there for over a year, I witnessed many things that really bugged me from members, but I won't even get into that as the vast majority of them don't know any better. At least they're in the gym trying to get better. In my opinion, that's much better than sitting at home doing nothing, even if your workout sucks. Admittedly, I wouldn't say everyone in the gym needs a personal trainer, but most, if not all average gym-goers could at least use a friend/mentor who knows what they're doing to guide them along, and tell them to get off the BOSU ball and machines and learn to move more efficiently and do things that actually work.
My biggest complaint with commercial gyms is the trainers and management. As a trainer myself, I take great pride in what I do, teaching my clients proper form/technique and designing programs based on their needs. This is what training is all about. However, over the year I spent training at this commercial gym, I probably had the pleasure (if you want to call it that) of working alongside somewhere in the ballpark of 30-40 other "trainers." Sad part is, of all of those trainers, I could honestly say only 1 other knew what he was doing and actually took pride in his job. Out of 30-40 trainers I knew, he's the only one I would ever recommend anyone train with. The rest of the trainers fall into 1 of 2 categories: 1) they have no clue what they're doing and probably need a good trainer themselves, or 2) they simply don't care enough to actually coach their clients and give them proper programming. Even worse is the management that allow their trainers to perform so poorly, exposing clients to risk of injury. At the gym I trained at, not even management knew the first thing about training, so I couldn't expect any support from them. People are paying good money for training, but sadly, would 9 times out of 10 be better off flushing their money down the toilet.
Despite all of this, I will say it was a great experience for me because it exposed me to a wide array of clients, made me want to be better, and reminded me why I got into this profession in the first place: to provide people with safe, effective, and proper programming to help them achieve their goals. This simply was not happening at said gym. Which is exactly why I recently left with a friend to open our own facility where we have 100% control over everything that happens and can ensure people are doing things correctly. There are some great trainers at commerical gyms, but they are few and far between.