I don't always use a 50, but if I'm carrying primes in my bag, a 50 is always one of them. Sometimes this is exactly the focal length I need, which is why it's always in my bag.
A couple of things to consider that most folks don't, I'll wager. The 50mm is most likely the best optically corrected lens in ones collection. A good 50 is usually an APO lens, although seldom, if ever, indicated as such. It's also usually the fastest. So for these reasons alone, it's always a good idea to keep a 50mm within easy arm's reach.