I wonder if it's possible to launch an orbiter in Mars synchronous orbit. That'll eliminate the bottleneck.
[1] http://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter
The current orbiters are primarily for imaging, so it makes sense that they're as low as possible. If you were always going to have several surface missions, it might make sense to have higher orbiters primarily tasked with communication relay.
One thing is that they didn't know Gale Crater was where they were going to land until pretty soon before launch. There were a number of other sites. And if they chose one further from the equator, that satellite wouldn't necessarily be in the right place.