It's a concession a lot of them make because they feel like they can move better without it. At the end of the day they get paid to perform, and the better they perform the more they get paid. For a skill position guy they may be willing to deal with some things to get a boost in performance. Truthfully a knee pad isn't really going to do much when you get a helmet to the knee. All part of playing the game.
I thought of the freedom of movement idea ( and it makes sense ) but I also know that once you start playing you don't really think about ur equipment being on . I used to wrap tape around my thigh pads and knee pads so they wouldn't shift around , and know they made a difference when you get hit in those areas cuz it does cushion the blow some ( though it wouldn't be helpful with the awkward types of hits ... the bend wrong / get injured types ) - but they're still somewhat helpful , so I think these guys are stupid for allowing more dings than necessary . Less pain = you feel better ... and every little bit helps . To each his own though .
Yeah...I don't quite get some of it either. I know I was very particular about my stuff...I used to cut down my hip pads because I thought they were obtrusive, and I used to roll up my pants a bit so I could bend my knee better, and I liked my shoes and shoulder pads tight. I guess these guys figure they're gonna get shot up to feel good so they may as well have their equipment the way they want it.