It can be a pain in the ass sometimes, but I'm certainly used to it, and it has its upsides.
It is likely that it, in conjunction with not being able to build under your feet, reduces building-spam of tall, ugly structures, and maybe other forms of building-related griefing. I like that someone can't just log in, gather material for a couple hours, then put up a tower to 255 in 10 seconds. If you want it, you've got to work a little harder at it. People might say it's not that big a barrier, but I don't think I'm underestimating how lazy some people are ("If I can't do it in ten seconds of mindless effort, I'm not gonna bother"). Good. We don't want those people. I don't, anyway, and am willing to endure the inconvenience as a trade-off.
I personally don't find it very difficult to employ scaffolding or to build one floor at a time. It adds a couple minutes per floor at most. I can see how it might be more problematic for others.
I appreciate the spirit of this mechanic, and there might be a more interesting way to implement it somewhere between turning it off and leaving it on. I realize that any such middle ground would make more work for Int, though, and if off/on is the easiest fix, I understand. Anyway, some possibilities off the top of my head:
One block or type of block that can be built midair/underneath, which would increase the utility, if not value of that block. Once those blocks were in place, they could facilitate building, then be swapped out later with the intended blocks. So, kind of a hybrid between scaffolding/placeholder blocks and free-for-all.
One kind/category of block that, when you go more than halfway off it, you're still considered on it, and can place anything in midair.
You can build midair if there is already a block underneath the block you're trying to place (This would add convenience to building while preventing the making of no-effort, 1-block bridges across chasms).
You can build midair if the target block has at least one matching block adjacent to it (again, would add convenience to building structures, which tend to built of the same thing, but limit other uses, such as wood-planking across a stone/dirt chasm).
You can build midair while under the effect of a potion, golden carrot or golden apple. It would certainly give us something to do with gold.
Or wearing boots with Feather Fall.
If the choice is off or on, I'd just leave it on. If it stays on, I'm willing to share scaffolding techniques. If it gets turned off, I'm sure I'll get over it.