Two spaces after a period is absolutely NEFARIOUS. The devil, I tell you!
I know people who do this, and I love many of them. But reading such copy is a mental case of the hiccups. Furthermore, editing extra spaces out of every sentence is a giant hassle, and is something that not even a group edit will always fix in full.
But what I like least about the practice is why people do it: more often than not, it’s because it’s what they were taught in typing class back in the typewriter days. It made sense back then, in the age of monospaced fonts, but it’s the computer age now. It’s just plain stubborn to stick to that.
Doing something strictly because it’s the only way you’ve ever done it is one of the worst reasons to do anything. There’s comfort in routine, sure, but sometimes it’s worth it — both for others and for yourself — to evolve.
Or, at the very least, break out the old Smith-Corona, which would be pretty cool.