From my experience in the beta, it seemed like it was overly difficult to reinforce a siege. The outcome of almost all of the sieges I took part in was decided on the outer walls of the keep. If the defenders managed to hold off the attackers outside the walls for a while and kill some of them, the attackers were forced to retreat. But if the attackers managed to kill some of the defenders and broke through the walls, they were pretty much guaranteed victory if their group was large enough to deal with the NPC defenders.
As a result, almost all of the sieges were over quickly, lasting somewhere between 5 and 15 minutes. And there was very little PvP to be found inside the courtyard or the keep. A very big part of the reason for this seemed to be that it absolutely takes forever to get back into the fight if you die; it's a very long walk from one keep to another. And the reinforcement tents (with limited uses) were very expensive, so they were almost never used.
If I compare the respawn options here to those in Planetside 2 for example, then that game seems to do a lot more to keep sieges exciting to the last second. Defenders there can keep respawning near a "keep" until certain structures have been shut down by the attackers, and these same structures can be recaptured/repaired to be used again if the attackers leave them unguarded.
For the attacking force, the primary tool to remain in the fight were Sunderers (mobile respawn tents, if you will). They didn't have a limited use like they do here and were a lot more affordable, but were destructible to keep them balanced.
I think it may be beneficial for Cyrodiil keep sieges if respawning/reinforcement wasn't as restricted as it is now. Perhaps some of the resource nodes surrounding a keep could be used as respawn point for people who died in a keep's vicinity, without being connected to the teleportation network between keeps of course. As a bonus, it would open up some strategies for a defending force to bust out of their keep to reclaim a place to respawn from the attackers.
In one article from the press, I read they were glad it was so hard to reinforce because they didn't like long sieges. But I personally prefer them, especially if the odds can frequently shift back and forth. I'm curious to hear what other peoples' opinions on this subject are.