In an ideal world the Help Desk team would have dedicated technical resources to solve user problems only. The problem in most organisations is that the technical resources for user problems is also the overall maintenance team. The issue then is what takes priority.
Where I work it is very simple, product issues come first, then user problems and then only general maintenance. for this reason the two are separated, the level of the incident is controlled and the help desk will report back to the user when they can expect support, but they cannot set organisation wide priorities and decide when a call will be seen to.
What makes it logical to work this way is the issue of pressure from users for their problems to be resolved immediately even though organisation wide it does not make sense. The separation means that the help desk staff have a measure of protection from this pressure.
Cheers
Marc