The only people you would be in competition for with gear drops would be people whose first main or only main is a shaman.
When I think it through, Billinias as an alt gets second crack at all the shammy gear that drops on the current raids. But all the current raids are on farm so for the most part, all the elder shammies have the gear so it would automatically, for the most part, be offered to alts. The only people, without you switching mains, who would get higher priority would be a main that hasn't been on many raids yet, or an elder main who has been looking for a specific drop for a long time. So you want to gear up a second main at the same time somebody is trying to gear up their first or been looking for a specific rare item drop.
I know you'd do the right thing and pass drops to those people, which again, puts you to second fiddle. So I am confused about your switching mains.