2 chokes
Easy, quick cleaning
No hulls spitting at the next guy
Easy hull retrieval for reloading (if lead goes down again, anyway -- haven't reloaded in months)
Easy takedown transport
Better balance
Safe range carry (unless you run into that jerk in the thread a little while back who threatened to shoot people who carry an O/U in the accepted manner)
Subjectively, a semiauto usually feels like a rather unwieldy contraption compared to an O/U of the same barrel length.
In 12, I have accumulated one of each (pump, semi, O/U). The O/U is the only one I've used since I bought it, and the semi is the first one I think I'd sell, except that it may not fetch enough anyway. I like a 20 Gauge O/U in the field, too: quick, shorter than a semi, 2 chokes, takedown transport, can't jam it by shoving in a shell backwards, easy cleaning, safe carry around others. Never even got a 20 Gauge semiauto, got hooked on the O/U, which I picked up used for 500 bucks, as soon as I used it.
That's my personal preference, but it's a preference that comes from trying stuff, not reading marketing materials. I'm also picky about what O/U I like; i.e. I'd rather shoot my cheap, rough 870 Express than a really nice Citori for anything but trap, because I find the 870 to have better handling.
Particularly now that semiautos are hardly a budget-conscious alternative, a semi is the last gun I'm interested in getting, and not just for the range. Maybe if I move to a place where I don't have to plug an upland gun, and there are so many birds everywhere that it matters, I'll want more shots. Maybe. I don't much care for semiautos, though. The Berettas are pretty nice, and the Benellis if I close my eyes, but they sure feel too long for anything but a goose gun.