A little off topic here, but I've also seen guys bench with their feet on the bench. What is the purpose of that? I've never gotten the nerve to ask them why they are doing it!
There's 3 main reasons they do it. The most understandable one is that they don't know mcuh about it and someone that they trust told them to do it like that. Another reason is to further "isolate the pecs", by taking the legs out the movement (although, you don't need your feet up on the bench to take your legs out of the movement...um...just don't use any leg drive?).
And finally what ellerbus said about taking the arch out the lower back. That theory decades ago is understandable but not now... When your lower back is 'neutral' it's actually arched slightly. By flattening the lower back you actually begin to flex it (round it). This also screws up your stability in the upper back by changing spine position and affecting how the shoulder blades sit (hikes them upward a little), and of course, the shoulder blades stabilise the shoulder joint during benching. So, you actually make things worse, injury wise, not just on your shoulders but on your lower back, too.
And yes, benching with feet in the air is one of the most bizarre things ever.
I don't think much tops the agility ladder. The trainer get's the ladder out up where I do my warm up. Imagine paying someone for an hour to get you bulked up. Then, they spend atleast 5 minutes unrolling the ladder and setting it up, and they do so with such pride - like a guy who has, i don't know, model ships that he's built, and handles them with such delicacy and care. This is the trainer with the agility ladder. And she's not even agile, she's quite heavy footed. You'll hear her saying, "ok, Ali shuffle" and you think, "cool, someones doing the Ali shuffle" and turn around, and, well, it's more like a drunk guy at a wedding attempting the moonwalk. They spend about 20 minutes doing it, plus 5 (atleast) setting it up. By the time they go and do whatever bag of tricks she has in store next, it's basically half the session.
Ach, the joys of commercial gyms.