Penguins at a zoo...
So many reasons to have video, then why not? Any good reason not to have it?
Adding cost? Probably not much we had the live view already so it's more an incremental cost.
Purists #1, it's a camera not a video camera. Don't use the video mode then

, besides it's not marketed as a professional camera, but it's going to be improved and I'm sure we're going to see it on all models eventually..... and if it's really successful we might see a dedicated model just for video, who knows. Yes it's not a video camera, and it won't do certain things a video camera can, but it will do things a video camera can't.
Purists #2, if I you want video, bring a video camera. But a lot of people don't want to bring a video camera. They want great pictures with the convenience of video when they need it. The people who want better video will get a video camera with stills capability. They'll get their video but have not as nice pictures. Just depends on where your priorities are. This just gives everyone another option.
Purists #3, Why didn't they bring down more pro options from the D300. Hmmm... well the market for this camera is still more consumer than pro. I'm sure their marketing/development team looked at which options would be used more by the consumer than the pro and selected which options they would offer on the D90 within a certain cost.
Purists #4 I want a simple camera like an FM. Develop a digital FM maybe called a DM with manual shutter, able to meter with AI lenses, no auto modes, no autofocus, etc.... Why waste money developing that when they could add AI metering to a D40 and be pretty much there for digital slrs. If you really wanted something manual that could shoot without batteries then they could add solar panels or use the film advance crank to charge the batteries needed for the electronics, just no LCD in the back

.
I have a Canon Vixia HV30 and the video is great and the stills are actually pretty good for a video camera. If I was shooting a p/s camera I'd probably just bring the video camera instead of both. With the D90 I'd probably leave the video camera at home. That's what happened to our old video cameras when we got a p/s with video that was good enough for us. Convenience won out at a certain point.
Bottom line this just gives everyone another option which is better than having no options. Sorry about the rambings... I get this way when I don't get enough sleep

.