Yes and given the V8 license Safari could pickup V8.
Personally I expect V8 and SFX to share ideas over time possibly ending up with a single JS engine in the WebKit domain. Chrome has unique-ish operational model that makes V8 a better pick for them at this time.
Well competition always helps, but I would like to see things shake out in such a way that there are really only 3-4 browsers to test on. 3 browsers, 3 platforms to test on (sorta, FF on all 3, Safari on 2, Chrome on 3 eventually, etc), its a lot of VMs...