The same way I think of all these music games, they are limited by their catalogues, of which you would probably only like a pair of them. Same thing happened to me last gen with Guitar Hero... but I was able to hack Rock Band 2 and put some of my music into it.
The game&mic were selling for half price of the mic alone at other shops. Got two nights of fun out of it, so I think it was worth it. The downer like you say is that it one doesn't even have the option for DLC.