The current playoff system effectively extends the season - if it wasn't in place, a lot of clubs without a mathematical chance to catch the top 3 would lose interest.
Up till last weekend, even Millwall down in 16th spot could still have snatched a playoff spot (however unlikely), and now could easily go down just a week later.
There is a very good argument that the team finishing third over a 46 game season should go up automatically, but it would kill the hopes of some clubs who hang on to the belief that they might, just might, snatch a coveted playoff slot for a chance of the big time.
It's still there with just two games to go. If it was an auto for the third spot, Bolton, Leicester and Forest would be out of it, and even bloody Brum in tenth still have a mathematical chance!