And then Sonic Heroes's story happened and as loath as I am to admit it, at least it did
something with Metal Sonic. Episode Metal was incredibly boring and unbalanced. It wasn't much of a lock-on game at all as advertised, and I don't know where they were going with that.
The Sonic 3 & Knuckles, and Knuckles in Sonic 2 lock-on features just showed more depth with regards to the level designs or different pathways you can take with all three characters. The level design, in the cases of those games, was just demonstrative of how versatile it was.
Episode Metal by contrast is just a retread of levels from Episode 1 in an unbalanced fashion.
And,
someone does have Sonic 4: Ep 2 on their GotY list.