If a club comes to an agreement with a player, then changes their mind. Making the player see out their contract, in whatever way they see fit, all the work they have done improving the prospects of the club, are taking a huge hit.
Some posters are letting their hearts overrule their head. Imagine you're a talented footballer and your dream is to play for City.
You're offered the chance to join a club, that took the step of forcing a player to see out 3 years of his contract, when he declared his desire to leave.
You are also offered the chance to join Aston Villa. You are destined to be the star player but are on a 5 year deal.
In that scenario, which club would you rather be at, when City come calling 2 years into your contract?