Fighting spam is an important issue, and isn't just a problem MMO players are faced with. However, when implementing measures against spam, you always have to weigh how much those measures actually hinder spammers versus how much they impact legitimate users. If your measure impacts legitimate users more than it does spammers, it's very likely a very bad idea - and region locking servers would be such a case.
There's way simpler methods SE could implement to fight RMT spam, and which would not or only very slightly affect legitimate players. One, for example, would be to give players better control over who they want to be able to receive /tells from (currently, we only have the "nuclear option" - disable all tells with /busy). If you could select whether you want to be able to receive only tells from Friends, your FC, people in your party, the Novice Network, and/or from people in Linkshells you are in (ie, a whitelist approach), this would already go a long way to basically remove RMT /tells without impacting legitimate players in a noticeable way.
As a side note, I don't know how bad it is on other servers, but Cerberus has, at least in my experience, barely any RMT spam currently. It used to be worse, but nowadays, I get an average of one or two RMT tells a week, and usually at like 4 in the morning when the server is pretty deserted anyway. The only other common RMT spam I see is the lone bot who hangs out in front of the MB in Limsa and drops a line in /say every few minutes, which I barely notice anymore.