I wouldn't say lots of people since I've never once downloaded a real book from a real publisher who used anything bigger than H3. There probably are a few, but I'd say they're rather rare. The only kind of titles I can imagine it working for are those that simply have a chapter number and no other writing. Just a great big "1" etc. Here's a few examples. The first one I did using H3 titles:
The next is from Azimov's Foundation novel I downloaed from Amazon last week. It seems to be H3 also.
This is from a novel by Eric Frank Russell called Wasp I just downloaded from Amazon last night. Seems also to be H3:
And last is one is a Sherlock Holmes book I got at this website about a month ago. It's probably H2 I'd say:
Aside from the fact that they screwed up the line spacing and left no spaces whatsoever between the chapter title and the text, that H2 font sticks out like a gaint in the land of fairies.
If you have an H2 or H1 font size for titles, many of them are going to run two lines--not very professional looking. For people who are just making books for themselves, I guess it doesn't matter, but if your goal is to make sellable books for download, I think it would pay to keep in step with what the industry is doing, and I don't think you're going to find many big-time publishers using huge H1 and H2 fonts in their titles. Yes, I can change it to whatever I want, but it's yet another step I have to go through. It would be a hassle for someone just learning the program. That said, it seems to me that if Calibre wants to be taken seriously by industry professionals it ought to stick to industry practices and make H3 and H2 the default for TOC titles like nearly everyone else rather than H2 and H1. It's just my opinion of course. But H1 is just huge. I can't imagine using that for anything.