It's always good to have a back-up or to have different lenses at the ready.
The GH2 definitely affords a better grip and is also quicker to make changes in focusing mode, etc. The IQ actually seems pretty comparable, though I haven't done any objective testing. Both are better than my other bodies (G1 and GF1).
When I'm walking through a park, I might have my 100-300 on the GH2 and either the 45 Macro or the 7-14 on the G3. Inevitably, when I'm walking around with anything other than the 100-300 mounted, I run into something that demands it immediately.