As the author of an application that supports AppleScript, I can tell you that implementing it is a difficult, thankless task. Apple’s documentation is poor and confusing (what else is new), and they really only provide a minimal skeleton for you to build on. So it’s no wonder that there wound up being so much variation between implementations. And since so few users will actually use AppleScript, developers can’t really afford to pour significant resource into it. As I mentioned before, many developers just throw up their hands and don’t provide any AppleScript support at all.
The original question was whether it was worth it to learn AppleScript at this point in time. If you need to build an automation solution that ties together multiple apps, it’s pretty much the only way to go (assuming the apps you need to use have scripting support). It seems like Apple is probably not going to drop AppleScript any time soon, just let it coast with no investment or promotion. There probably won’t be many new apps that support AppleScript, but there aren’t many new Mac apps period. It doesn’t really cost Apple anything to let things continue, so I anticipate the status quo to continue. AppleScript can’t be easily replaced because doing so would require the participation of all the developers.
By the way, I recently added x-callback-url automation support to my application (so it could talk to apps like Ulysses, which support this but not AppleScript), and AppleScript doesn’t look so bad compared to x-callback-url. (However, my hat is off to the x-callback-url developers for figuring out a way to make interapplication communication possible at all on iOS, without access to the internals of iOS. A very clever hack!)