I prefer what Avon from Blake's Seven said on the subjects, that a pilot is a dime a dozen but a good thief is hard to find.
You gotta remember that all the Starfleet officers, in theory, even the knuckle dragger's in security are super geniuses who can all do Tom or B'Elanna's job with equal textbook potency.
Point is that B'Elanna and Tom are supposedly artists in their fields, even though could easily do each other's jobs competently enough that the ship would not blow up.
Neelix is a shitty thief. but then among the Maquis is probably the thief artist gandering in obscurity.