I never do the side missions, its a roleplay game so I roleplay, given that the galaxy is in imminent danger it wouldn't really be within my role to go off and find someone's missing data disk or a missing soldier or whatever. The galaxy is in danger that's what I would be focusing on, not needless diversions which only delay my chances of saving the galaxy.
Maybe, but if you do that people die!
There's at least 2 side missions in ME2 that should have a big impact in ME3, and there's a decision in ME1 that would also seem to have a ramifications for ME3.
What I'll be interested in seeing is if the DLC addons like Lair of the Shadow Broker will have an effect on ME3.