No, no... on Hoth, the shield was for defense from orbital bombardment.  It didn't have to come down to the surface, which is why a ground assault was required.  Land beyond the radius of the shield, move in and blow the generator.

A better concern would be, why didn't they just blow the generator and then retreat... letting the fleet obliterate the entire Rebel base?  They'd have caught a lot more Rebels on the ground, that way.  Egos, I guess.  The army wanted a major victory on the ground to rub in the fleet's face.  Vader wanted to land with his troops to sweep through the place and take prisoners.  The wampa would rather kill a few more people before dying than try to escape.

Edit: I just remembered why Vader would want to capture the base.  It's because the Wampa was captured.  He just wanted the $100.