My argument for no second serves goes like this: spectators generally prefer to see long rallies than aces or service winners or short points caused by big serves. There would also be more action and less buggering about.
Against that, as Phil says, courts are being slowed down to cater for this and it's killing the players. So maybe now is not the right time to introduce one serve. I think the perfect combination would be faster courts and no second serve.
It just wouldn't work.
1) It would mean the players would end up having just one serve - a powerful one that results in a shitload of faults or a weak one that is a liability, not a weapon. They would have to work on something in-between - relatively safe but still strong... and you'd get the worst of both worlds.
2) Serving would no longer be an advantage, and breaking serve would no longer have any relevance.
3) Way too many faults. They are confidence-sapping and not fun to watch.
There are probably plenty of other obvious points but having two serves is deeply ingrained into tennis and there are plenty of reasons why. It's also quite damn satisfying as a fan to see a player fire down an ace or unreturnable serve, and the short points offer nice variety to a match (when the player's not a serve-whore anyway). Returning is an art, and the balance between good servers and good returners is fine.
I'd like to see them do away with lets though. Whatever happened to that trial run in the Challengers? Way too many points get replayed for a serve that barely grazes the net and clearly has no impact on the serve.