Imagine that your kill team is going on a mission where you'll want to have a missile launcher/ las cannon/ multi melta or some similar gun.
The obvious character to give it to is a devastator if you have one in your kill team. However in my games it typically goes to the apothicary, the librarian, or a tactical who has the command special ability rather than the bolter special ability.
The reasoning that goes on is this: Our devastator has this pretty darned great heavy bolter that saved our rears on the last misson. We need to requisition a big gun for this mission but lets give to this other fellow, that way we get to have the mission required weapon and keep the heavy bolter in the kill team.
Has this been your experience as well?