I'm pretty sure most people do NOT use oUF, at all. It's going strong here on wowinterface and only because it's the "hometown" of haste + oUF. Talking about unitframes in general with people - like those from my guild full of about 300 players, that play wow for 6 years - never even heard of oUF. That reveals/represents more of what's actually going on in terms of most people's interface/unit frame choice.
This is a damn pity, of course. oUF should be used way more, imo. But people want to "customize". That's basically the main concern of most. oUF means getting into lua a fair bit (even if it's just customizing existing layouts) which equals to mass incompatibility in the first place.
Typical oUF layout comment: Looks nice, BUT how do I change "....."?!?
OT: While a HUD can fit into an UI, I still don't like them. But that's personal preference. First question that comes up in my mind when I see a HUD UI: Why does it have to be so god damn huge? See, it's personal preference again. You can't argue about that.