Setting a limit is fine but it does seem counter productive from a marketing stand point. I'm not sure anyone thought it through before saying "Once it expires, they will go buy our game!".
Case in point, my kids kept playing the Sonic and Mario Olympic Games demo and repeatedly asked for it for a month. Once the demo expired on them, they completely forgot about it. Now it's Christmas time and it's not even on their radar anymore. If it was still playable, they would probably have had it on their list. Also, my kids love showing demos to all of their friends when they come over if they think the game is cool. They do that with the Xbox all the time, but there is no way they are sharing the Rayman Legends demo since they have limited playthroughs to last them until the game is released.
So I think a better analogy is to say, check out our commercial ... but you can only view it 30 times even if you wanted to see it more.