Well, properly used, a booster is very safe in frontal impacts. In side impacts (much rarer, at least), is when things get a little less clear with regards to safety (which is why the government is mandating side curtain airbags soon...too many dummy's heads flying out of side windows, unfortunately)....but presumably a child in a booster is at least as safe as an any adult in the vehicle, as long as the lapbelt is on the lap, and the shoulderbelt is on the shoulder.
What seat do you have? Maybe a lighter harnessed booster would at least be a little easier to tote around, and you might end up using it as a spare a lot. The Graco Platinum Cargo is nice, as is the Evenflo Chase or Vision or Traditions (same seat, different cover options...and definitely sits up on it's own when you put it on the ground...not the worst idea for a travel seat, I used to have a Cargo, but can't remember if it did that).