This doesn't really bother me. The extras aren't much and this game has been out on the Cube for so long that if, for some weird reason, someone was willing to buy a Cube for it they already have. A port this late doesn't concern me. Odds are it won't sell very well anyway (delayed ports rarely do) and that would mean the Cube version sold best which is always a good thing.
As an owner of the Cube version I personally am glad they didn't add Sonic CD. Still from a game purist perspective it bugs me that they had a chance to "fix" this collection and make it definitive and chose not to. Sonic CD should be on the collection. The ideal scenario would be if they had added it to this version and then offered a standalone Cube port of Sonic CD as a mail order or something. Or make a Mega Collection 2 for the Cube with Sonic CD, Knuckles Chaotix, Sonic R, that Sonic fighting game, and the Sonic arcade game.
Was anyone besides me upset that the Genesis version of Sonic 3D Blast was included instead of the superior Saturn version? I mean the game sucks anyway but from a historical perspective the definitive version should be the one included.