I play pretty casually and do a lot of pugging and right now I'm up to Twintania and Ifrit Extreme. It all depends on how you use your available time, although I do try to set aside at least one night a week to raid with my FC, although it doesn't always happen.

I'm not sure what's preventing a casual player from joining a FC or using voice chat. There are plenty of FCs out there that play casually and it's sometimes nice to be able to ask familiar people to play with. Voice chat is just a quick download and you're set to go. You don't even need to talk, just being able to listen can improve performance greatly and it allows people to relay information faster when they can't afford to type during battle.