I bought a mech very recently for the first time. Why?
1: It is aesthetically pleasing to me, compared to every regulated box mod that looks exactly the same.
2: It is durable. I can drop it in a pot of boiling water, dry it, and have no issues using it. I can also get replacement parts easily and cheaply if something should happen to fail.
3: Clone mech mods are cheap. Really cheap. You can spend 10$ and get a functioning unit that will work very well. You can spend 30$ and get a functioning unit that will perform extremely well.
4: Why can't I have both? A regulated device and a mechanical mod. I see no reason to corner myself when both work for me.
Yes, the battery will drain while using it, and will need to be changed out when it is time. That is, for me at least, the only major caveat and drawback compared to a regulated high wattage device. Still, for me, it isn't a big deal; I have batteries to swap in when it's time.
Mech mods aren't as popular now because of the availability of high output regulated mods, true. Regulated mods are generally more idiot proof as far as safety goes. Mech mods require more diligence. If you don't want a mech mod, or the responsibilities that go with it, don't get one. Personally, I love the form factor, and I'll probably be adding a few more to my collection, along with one or two more regulated box mods.
I can't answer "how often" to change your batteries. That depends on your usage.