My potential uses for HDMI:
When possible (either at home or at a friend's/relative's house) I feel that showing pictures on a TV is a much better option than on a handheld device. Many high end cameras (the DSLRs that are actually there for stills instead of trying to be movie cameras and phones at the same time) have an HDMI out these days, having upgraded from the old RCA video out.
I have my entire DVD collection on a DLNA server as well as PlayOn for Hulu and NetFlix support. Previously I didn't bother with getting TVs for certain rooms since I'd need an XBox 360 or other DLNA-enabled media player with a whole new interface that I didn't want to manage. Now I'm thinking I can just plug the Droid into the TV and watch whatever I want, and how cool is it to have the Droid as a remote that's showing what's on the screen as well as giving the ability to use the control buttons? Now I just need to run an HDMI cable from the TV to wherever the viewing area is.
I'm curious to see if I can hook the Droid up into the projectors at work. If I can run a presentation from the Droid I may not need my laptop. Since most projectors these days have at least a DVI input I have hope that I'd be just a cable away from this.
Recently I was at a party and tried to get a neighbor's PC hooked into their home theater setup. We ended up hooking into the projector behind the room via the VGA cable and then running a long 1/8" extension so we could plug it into a portable radio at the rear of the room because we couldn't reach the audio system. It would have been so much nicer if I could load the video onto the Droid and just plug the HDMI cable into the receiver.
The big question is what will be HDMI-able? I've seen some specific examples of what is, some of what isn't, a generic statement about "non-DRM video" but I'm not sure what the API allows now nor what it can be expanded to in the future. Even if there's no PPT viewer today that can output to HDMI there may be an upgrade as more phones support it.
Would I use HDMI daily? Probably not. Would I use it frequently? Once I add a TV into the bedroom and the exercise room to replace the aging laptops there now, I think I would. For me HDMI was actually the driving factor between the X and the Incredible, I'd like the Incredible's "hackability" but I see a lot more potential in HDMI than I do in getting Gingerbread a few months earlier than Verizon would let me.