If your goal is to get someplace, turning around and going back home
may not be very useful, but it's certainly legal.

It may be very useful. If you have a "there or nowhere" destination -
meaning if you can't get in there, you might as well never have left
home - and your home is a valid alternate, it makes no sense not to use
it.

Whenever I make a business trip from Houston to Austin when the weather
is iffy in Austin but not in Houston, I file my alternate back in
Houston. If I can't get in at Austin, it does me no good to get in
somewhere 50 miles away with no arrangements for ground transport. I'm
going to miss my meeting anyway, so I might as well turn around and go
home.

Michael