MIG / TIG Combo Work - Makes sense? (Aluminum)

I would think that would work fine. However, you need AC TIG to do aluminum and that doesn't tend to be very cheap. You also have the aesthetics to think about. MIG and TIG welds tend to look very different so depending on whether people can see the welds, it might not look as good to have the weld different at the ends. Or maybe, it will actually create a better look to do it that way? Who knows.
And if you do anything but the thin stuff, you need a lot of amps for aluminum, and again, high amp A/C TIG machines don't tend to be cheap. It would be a shame to spend a lot on a low end machine which would work on the skin of a boat, but not have enough amps to do anything with support ribs or corners braces etc.
If I though I would be doing a lot of aluminum work, I'd get a TIG machine for sure even of the bulk of the welds were going to be MIG. I can fill 1" holes in aluminum with TIG but I would never want to try it with MIG. If nothing else, TIG would be nice to have when dealing with problem spots or to patch up errors.