They say every other day because we know for a fact that sperm can live inside the uterus between 3-5 days, so there's no need to really do it every day during the fertile period. Also, it may allow the man to build up more of a "reserve" if you will. But then again, this isn't to say that you will be any less successful if you do it every day.
I think it's important to relax about this and not calculate and think about it too much, because stress more than anything can have a negative effect. Keep track of your cycles and simply make sure you're doing what you need to being doing around that time....good luck