I use cycling water bottles because:

1) They are larger than pop bottles so I can carry more liquid with me

2) They've got tops on them which allow me to drink without unscrewing anything. Oh, and I don't use water bottles with "pop tops" - mine all screw on and are very secure.

3) The openings in the water bottles are much, much bigger than pop bottles, so I can get ice in there, and so that I can dump scoops of my powdered drinks in there to mix my drinks.

4) I've got a whole heap of them (free) from various cycling events that I've done, and am likely to continue to acquire them ... so why not use them!!