What would happen if you locked in landscape and then returned to the homescreen? Or any of the many apps (built-in and 3rd party) that don't support landscape? Apple has never stressed landscape support for the iPhone like they have with the iPad, so it would be awkward to allow the user to lock the device in an under-supported mode.
That's a fair point, and I think you're addressing the precise reason it's not there. Nonetheless, there's so much benefit in landscape lock that it would IMHO be far more than worth it for the apps that do support it.
What I'd really like to see:
Make Landscape Lock a mode that forces landscape whenever it's supported and ignore it when not supported. (Simple to do. The OS just always returns status of landscape orientation whenever queried). This would force us to hold it in portrait for apps (or app modes) that don't support landscape, and would give us exactly what we want when it is supported. What more could we want. We'd have a perfect solution for using apps like safari, when lying in bed.
When you think about it, that behavior is quite similar to what we have right now with no lock mode. When you turn the iphone horizontal today, some apps (even apple apps) continue to display in portrait. That's a disappointing surprise just like the surprise you'd get when turning on landscape lock and using an app that doesn't support landscape. Seems to me that it's just barely more awkward than what we now (in one way) have but DOES totally satisfy the need where applicable. (And we'd eliminate the crazy flipping in those situations where you need landscape, which is MUCH, MUCH more awkward IMHO)
I do agree though that there's enough quirkiness that apple is likely to refuse to do it, given their philosophy of "Do it right or not at all"
Pete