In the era of the 45-man bullpen, there are only four skills that are really relevant to being an MLB bench player: being an above-average hitter lefthanded, being able to play shortstop, being able to play center field, and being able to catch.
If you can't do one of those 4, you're hosed as to making a roster. If you can do 2 of the 4, you're in good shape (and if one of those two is the hitting, you're a bit unlucky not to be a starter). If you can only do one of the four, you better hope there is nobody trying to make the same bench with that one tool who is better than you at any of the others.