For what it's worth, here's my 2 drachmas: I don't read sheet music worth a flip. I CAN read it, but not fast enough to play a song from it. When I took piano, I would learn the song from cover to cover and then play it from memory. I have forgotten most of the songs that I learned, but that's another story. In the same way, harmonica tabs help me pick up songs for playing on the harmonica that aren't already ingrained in my repetoire. I haven't played in a while, so I'd probably have to brush up on most of the songs that I used to play.
Part of the appeal of a harmonica, at least in my opinion, is that you can play it anywhere and at practically any time, so for that kind of impromptu performance, having songs committed to memory is the way to go.
The way that most tabs are written here, you need to be familiar with the song in the first place, as rhythm (note length) isn't always easy to express, so if you don't know what the song is supposed to sound like, knowing which holes to blow/draw isn't going to do you much good.
Most of the tabs I've contributed, I either wrote them down because I knew them well enough, I didn't have to stop and think about it, or it was a song I was trying to perfect, so writing it down as a tab helped me put it into memory.
Tabs are useful tools for learning songs, especially since it doesn't necessarily matter what key your harmonica is in. As long as the tab was written for a major key harp, you can use any major key harp you please, and the song will sound fine. Likewise if it was written for a minor key harp, pick any minor key harp you choose. Unless you have a need to be concerned with playing in a particular key, then you need to know which harp will allow you to play that song in the appropriate key. (Just because you're playing on a C Major harp, doesn't mean you're playing in C Major.
) This doesn't mean, however, that you necessarily want to play from a tab when performing, so use tabs as a tool to help you get the song into your mental database, so you don't have to use it when you decide to impress your friends with a rousing rendition of "Goober Peas."
Adieu! (Or is that, "Adoughnt"?)