Well i'll disagree with you on that point. All good personal trainers will do a fitness assessment of their clients and then design programs to suit the ability of the client and then build up on it.
Perhaps you could explain your logic in not targeting various muscle groups usng dumbbells for example, just because one might not have be able to push up their own body weight. Dumbbells come in various weights and why not slowly build up your muscles?
Regarding paying a qualfied professional to direct you (i.e. telling you to do push up etc, as you put it) - each to their own. Personally, i like having an authority figure guiding me and encouraging me to keep improving. Others have the discipline to do it themselves (i salute you people! congrats!). My own gym - the weights area especially is quite male dominated and with no real guidance not an area many females venture into. With even one lesson with a personal trainer, it breaks the confidence barrier for the females i've spoken to. So everyone has their own reasons for getting a personal trainer.
Ultimately, everyone chooses to spend money in their own way. Some splurge on clothes or make up or books and others on fitness...say for example personal training sessions

each to their own.