It is different because today you vote for an individual to represent you and who therefore are theoretically accountable to those voters. If the elected fail to vote according to their constituents wishes, but toe the party line instead, then they have failed in their elected role. Parties only presently exist from the tradition of having them, the power they wield, and many folks preference to not examine the worthiness of the candidate but vote on their general political standing, 'left', 'right', or 'don't know'. This makes for difficulty getting rid of them as hopefully we must eventually do.. But with AV the parties are effectively 'cemented' into the process, since voters then vote for a party not an individual; making the removal of these undemocratic institutions even more difficult than before, hamstringing future revolutionary/evolutionary change to a better system.