There's nothing preventing you from using a tower case to build an HTPC. There really is no rule/commandment stating that an HTPC must be tiny or look like AV equipment or a game console. As I mentioned earlier, the HTPC is not a one-size fits all solution. Isn't that the whole point of them? So you could have something that's tailor-made specifically for your needs - may that be an uber-tiny box mounted to the back of the TV or a tower running GTX 580's in triple-SLI or a rack-mounted server case with 20+ HDD's in hot-swap bays or even something else entirely different.
Oh I agree - the thing is most people consider in HTPC to be a smallish desktop style case that fits in your rack. I got sucked into this when I built my HTPC and have since changed my mind.
Go look at the kinds of cases they use in AVS forum - and you will see what I mean. It's all about making a PC LOOK like an AV component. Of course these same people frequently complain about overheating or hot tuner cards..
I was pretty stupid because I am one of the luckier dudes around. My g/f is even LESS picky then me when it comes to cases. I like black functional sleek looking cases that aren't made of plastic. This is of course because she is an engineer and so values functionality over form.
My point was that in truth the tower case does function better - even for the low power builds. Reason being its easier to work on and maintain - and its easier to add new stuff to as well. I have since switched - got a wheeled chassis and stuck a regular tower case with a full ATX motherboard on it.