This is not to start a flame war. However, if recent discussions on the subject are any indication, this should go well.
I can understand that Star Trek is not about war (even though war has been a main premise of one series and a movie), but war is nonetheless an element of the universe. Sure, the technology is more advanced (and therefore tactics must also be equally advanced) but the need for ground forces is still present in the 24th century. The only thing that changes is how they are used.
The Dominion war has shown us that large scale planetary invasions/occupations can occur. Betazed, for example, was invaded and occupied by Dominion forces. If that is the case, it can stand to reason that other planets, while not as signigficant as Betazed but just as large, could also have been occupied during the Dominion war (or in other points in time). So how exactly is the Federation suppose to defend their citizens (one of their primary missions) without the use of ground forces? Eventually, some sort of mass ground force movement would have to be conducted inorder to rest Betazed from the determined clutches of the Dominion. In this case, the only really way to do this is through discrete orbital/areal bombardment along with a significant ground force movement moving "door to door."
In this case, ground forces would fill a purely defensive role. Sure, they are assulting a planet. But they are assulting a planet inorder to free their allies from a extremely determined enemy bent on inflicting great suffering on their enemies.
And what about emergency relief efforts? The Federation probably has several agencies that can handle mass emergency situations. However, it could stand to reason that Starfleet ground forces could fill similar roles in extreme situations.
Additionally, keep in mind that ground forces of the 24th century probably share very little in common with todays military forces. Look at how the role of U.S. military forces as changed and continues to change. Think of just how different those forces will be two to three hundered years down the road.
Once the need for ground forces has been established, why not call them Marines? That name, afterall, fits the role of such a forces quit well: a ground force that is deployed from naval/starship units.