I like that we get XP for crafting, and I'm cool with it even if we get XP for crafting the same stuff over & over again. Like LostPony said, a person could be playing a weapon merchant type (which I'm kind of banking on doing, myself).
Where it gets tricky is how much XP you get, how easily you can find parts & put together mods, how much it all weighs, how many caps you get for each weapon, how high Gun Nut/Science needs to be to make stuff worth the hassle, etc. Simply put, we don't know all the variables enough to know whether it's something people can use to grind effectively, if it can be exploited, or if going around killing molerats & radscorpions is a better investment XP-wise. There's just no telling until the game's out.