The unibody metal shell of the HTC One was probably the best decision HTC made for the device, as it helped it gain a lot of popularity and attention, and more than it would've gotten if they would've made it from plastic or glass. Fortunately, HTC is saying that they will keep doing the same for future phones, too, such as the rumored HTC One Mini and HTC One Max.
The full metal body for the HTC phones didn't just appear all of a sudden for the HTC One. HTC has been working for years on getting it right, ever since the HTC Legend, which was mostly aluminum, with a plastic top and bottom on the back. They're tried more iterations since then, with the last one being the HTC One S, but they could never get it to be full aluminum, because there is this problem with radio waves passing through aluminum. So they needed some plastic parts to make the phone work.
They've never really said how they made it work for the HTC One, but I believe it works through the plastic strips on the sides, which means they must have the antennas on the sides now like the iPhone had them since iPhone 4.
However, unlike the iPhone 5, the HTC One is much closer to a full aluminum phone, because the iPhone still has the top and bottom parts made of glass, most likely for the same reason previous HTC aluminum phones used plastic for those parts. It can't be just a design decision, because those parts are the most vulnerable to cracking when falling down, so I have to believe Apple did it because they had to, not because they thought aesthetics beat functionality, especially when they are the ones that have to repair them afterwards.
So HTC seems to be ahead of the industry right now with this type of build quality for its phones, and it makes perfect sense for them to try to take full advantage of it, and try to use it for most, if not all, future phones of theirs. The surprising part is that even though they are using this more expensive material and manufacturing process, their flagship phones are still cheaper than most of the competition.