Also teams don't realise how bad it is long term to park the bus in one off games. By playing your normal game against better side helps you learn a bit about the strengths and deficiencies you have as a team. It provides you with invaluable experience as to how to play at a higher level (like City's first couple CL seasons for example). It provides them with another game to improve themselves and become better equipped with the manager's demands. Sure, you might lose to a team like City playing that way, but as we've seen this season teams are losing to teams like City parking the bus anyway. But playing nice football and losing might be really helpful in that next relegation 6 pointer you have being able to actually play football.
Look at Watford. They've lost this season to us, Chelsea and United, conceding a lot of goals in the process. But in each game they've played nice football and given it a go. And it turned out playing that way helped them beat Arsenal and almost beat Liverpool, which is more than can be said for the bus parkers. And look where they are in the table too.