Servers will usually have some restrictions on extra dimensions. The last one I played on (playing unleashed), had Mystcraft disabled, the nether and twilight forest portals were accessible from spawn but no one could have portals for them in their base.
Promised land portals were fine, even though I'm pretty sure I was the only one who bothered making one XD
That sound's like the usual MO for most server admins, but i've seen even seasoned admins get puzzled when they see their players are talking about a new dimension they have never heard of.
I guess we just need more clueyer admins...