First you have to get a mouse ($$$), then a keyboard ($$$), then a joystick ($$$), then when your all ready to use it and play some games, you realize you need a hard drive!!!!!!
Most of the time when you buy a CDTV you get a complete set with CDTV, caddy, keyboard and mouse; the CDTV console alone isn't worth buying on its own unless you need it for parts.
And no, a harddrive isn't needed to play games on a CDTV, it will do fine on it's own with CDTV games, and just fine with floppy based games by using an external floppy drive.
You are right about the A2000 being easier to upgrade, but it has it's disadvantages as well.