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The title pretty much says it all.
There are some valid arguments against trial accounts in MMORPGs, such as usage of trial accounts by gold sellers for spam bots, and uh... Well, I can't think of any valid ones beyond that.
Honestly, every MMORPG that requires a retail/digital purchase and not just a subscription to play -needs- to have a trial account option. To counter the aforementioned issue, just make a sort of "trial island" like some games have. This trial area would give players and idea of what the game has to offer but won't be connected to the global chat system. The key here to make this actually work well, though, is instituting adequate tutorials on the island so that new players don't need to ask old players how to do something (like how to set up diplomacy decks in Vanguard before they implemented their island >.> ). I can't even count the number of times my friends and I have looked at an MMO and said, "Hey, that looks like it could be pretty fun" but never tried it because we weren't willing to pay 30-50 bucks for something we very well wouldn't like at all. By offering trials, many of these MMO's could bring in all those players who want to give it a try, but just don't want to throw down the money on a game that's part of a genre whose notoriety for poor quality under the guise of greatness is second only to anime.
And to those of you in the "MMO's should not have trial versions," why do you feel that way?
Edit: For clarification, I'm referring primarily to launches of MMO's, in addition to MMO's that have been out for extensive periods of time and still don't have trials offered.