The main issue would be the dragon. The world itself wouldn't cause much lag, due to it just being a simple 50x50 world with one entity in it, the dragon.
As the dragon is flying around, when it runs into blocks, they immediately disappear, which CraftBukkit also raises events for. That can cause lag too. This dragon can also crash clients on older systems, due to how it works. At least this is my observation when Lynxy and I tested it out. At the time, we couldn't kill it with commands, so it just constantly destroyed blocks without us being able to stop it.'
Obviously we can control that now, but the reasons above are reasons we won't add The End.
The problem is also that it's pointless because it is only a 50x50 world that does take up memory on the server. Memory that could be used elsewhere. Sure, it may seem that we could add it and it'd work out fine, but there are reasons we don't add it.
Sure, we could block even the dragon from spawning in The End, but I'd also like End Stone to be rare.
So two reasons here, the Dragon, and End Stone. However, it might be open for debate sometime later.