It's hard not to sympathize with both sides here. It's true, the backend switch kinda sucks. Years worth of functionality and support, in the form of addons, just go poof. This functionality can be rewritten of course, but that will take a considerable amount of time - and that's the best case scenario. More realistically, it's a question of 'if', not 'when'.
When MM5 releases, most people have two options - make the switch and lose stuff they got used to for close to a decade, or stay with MM4 which will eventually lose support. As a user, you are pretty much stuck between a rock and a hard place.
On the other hand I can't blame the developers for making this call. MM4 is built on technologies that are outdated, deprecated or dead. Of course they knew that a hard cut would, on top of having to rewrite the whole application from scratch, break all existing addon support, but what's the alternative? Keep supporting a scripting language that has been dead for years now? It's a bandaid they had to rip off sooner or later, and that always hurts, no way around it.
Personally, I don't really mind that much. Mediamonkey is a great media player, with or without addons. I'm not sure if I would recommend start writing extensions just yet. Right now I'm running 12 custom extensions (two of which are public
) and it's very much like ZvezdanD said for MM4, stuff still gets changed around on a regular basis and it's likely that you will have to adjust your extension around these changes - especially if we are talking about skins. Seriously, don't write custom skins right now, it's not worth it.