My thoughts are that annual is the way forward; no there won't be the amount of finished, completed mods as in biannual events, but 1) take E3 as an example, rarely are there completed games, but it still excites and motivates and 2) taking qexpo 05 and qexpo 06 as an example, even with a year's gap there were still plenty of cool stuff on each.
I also think you tend to get people who have projects and release on alternate years, some with either more time, smaller mods or are harder working who'll release every year, and some who never get things finished but show unfinished cool stuff that motivates and inspires anyhow.
I don't think qexpo needs to be about finished stuff anyway. It can be about sharing problems, developments, ideas, tutorials and "how this was made?" type essays. It can also be more centered around events; speedmodding, mapping, matches etc.
I don't think the positive effect of qexpo can be underestimated in motivating people, giving people a target to get things done, in getting factions to talk and share more than they usually do.
I think that the problem maybe finding people to run it so regularly, as it must be a huge undertaking; each perosn over the years has always been so committed and so tired at the end.