That certainly was Apple's main intention behind iMovie 08: Forcing ambitious amateurs to FCP or FCE!iMove6 HD was good - they killed it when they redesigned it and I switched to FCP ...
Heck, iMovie 06 worked in certain circumstances even good enough for pros to edit a short clip on the run...
In 2003 the whole feature-length and award winning documentary "Tarnation" was edited with iMovie on a G4 iMac.