It has several uses besides ammo and weapon repair, can help for quest shortcuts and stuff, but otherwise it isn't too important, since you can repair weapons to full at any level now. Just put in a couple points every level.
As mentioned, everything that isn't ya bare fist usually deteriorate. A few number into that skill and/or a handful of that magazine Fixin' Things wouldn't hurt. Ya can also have someone repair ya stuff for a fee, of course.
As far as I have been able to tell, the repair skill only matter when it comes to crafting items from the reload bench and some items from the work bench. In this game, your actual repair skill doesn't do anything beyond unlocking recipes for these two things. Even with a repair skill of 16 I can still repair all my energy weapons up to full if I want. The only thing that actually effects the amount repaired when you do the repairs is the condition of the item you're using to repair it with.
That being said, I would advise that you put it at at least 50. It will make repairs that you do do for your weapons and armor more effective and you will be able to craft weapon repair kits. (which are awesome and will save you a butt-load of caps)
Also, if you're into looting victims to sell loot, repairing their weaponry (with a decent repair skill) will lighten your load while not really reducing the amount of money you can sell them for.