Even though I mostly use iMovie '11 (WAY better than '08 and '09, and also a bit better than the latest version), it's still good to have iMovie HD 6, as it can do some stuff the newer iMovie can't. But when you compare iMovie HD 6 to iMovie '11, the latter does have some neat features. Plus, I think the reason iMovie no longer has plug-in support was because Apple decided to just do some of those effects themselves for versions made after '08. ("Why get GeeThree Slick when our newest version of iMovie has picture-in-picture AND blue/green-screen effects?")
Plus, iMovie HD 6 is great for anyone using a PowerPC G4 machine with iLife '08 or '09 and doesn't want to bother with the hack for installing the newer iMovie (if the Mac is CoreImage-compatible). It was designed for use on G4, G5 AND Intel machines. Also keep in mind, some professional projects were edited on the classic pre-2007 iMovie versions (like the documentary "Tarnation," edited using iMovie 3 on an iMac G4!)