C2 can already do a lot without any plugin.
- associative arrays: a way to gather data (character stats, highscore, etc) as a pair of key,values you can access the value by calling the key. In some case it can be really usefull. That's why Mipey created a Association List
- canvas: with the default build of C2 you can't really manipulate pixels one by one (unless creating some 1x1 sprite with 16777216 animation frame (one by possible color) and spawning them where you want :D so yeah for that, plugin are usefull
Look at all the plugins there already are. By reading their description you'll see that they cover some intersting side c2 doesn't... yet (for some)