Liz,
I think the difference is that there are WAY more indoor guards out there than indoor percussion units in general, and especially in the Northeast and Mid-Atlantic states. When it comes to independent units (who generally take up more of a weekend than scholastic units do in practice time), there are way more independent guards than independent percussion units. That's why the DCA drumlines can get together during the winter for practice more than the guards can. And as you know all too well, in the mid-Atlantic region, indoor guard doesn't end after WGI. It's going to be the second weekend in May before DCA guards can really get going if they have members who perform in TIA.
The key is being smart with the time that you do have to practice and learn the show, IMHO.