There are a few reasons this is bad for the server:
1) Less people interacting at spawn because it's impossible for 1 person to be everywhere at once
2) Less changes to older structures keeping spawn "new" and "fresh"
3) Less diversification for newer players / older players that don't want to hop on every 30 days
There are also a couple reason why this is good for the server and its people:
1) Promotes trading of plots so that more people can create bigger builds within multiple plots
2) Gives new players the chance to snag themselves a spawn plot if it goes public
Anyone over the limit can decide which plots to disband and which to keep and I guarantee that almost every plot over the limit was purchased from a public plot. This way, people aren't really losing any money in the process.
My suggestion is simply to limit the number of spawn plots 1 person can have within a certain ring of plots. This won't completely fix the issue because obviously that person could just put the plot into someone else's name, but this requires them to do more work.
Example: I own 10 plots within 6 rings of spawn (10 is our theoretical limit). I find a couple public spawn realms that I want but can't get because I have already hit my limit (this could be enforced pretty easily). I would have to go out of my way to make sure someone from my guild or a friend gets on or comes over to that plot to pick it up for me therefore giving plenty of time for someone else to get the plot.
I don't know what the exact limit should be, but I would suggest something like 6-10 plots for the first 6 rings of spawn.
Here the number of plots owned by people within just the first SIX rings of spawn:
Finalmoe (RSMCYT): 12