My thoughts on the matter:
I am still fairly new to UO and Siege (only started playing less than a year ago). And one of the things that I enjoy about Siege, compared to prodo shards, is the fact that it is significantly more challenging (at least in certain aspects). I personally have no problem with only having one character allowed on Siege, and feel that it contributes to the challenge.
With only one character, you kinda have to adhere a little more strictly to certain templates. Its not like prodo where you can have 7 fully built characters all ready to go at will. This is what contributes to the difficulty (which, in my opinion, is one of the points of Siege). I understand that not everyone wants to play the same template all of the time (I myself have a handful of soulstones), but that is what soulstones are for. You dont have to be a vet to use them, and while they do cost $9.99 from the EA store, there are other methods to get them (I havent payed a single cent for any my soulstones). The only thing that makes changing templates not quite as feasible is the daily stat gain cap. I think it would be much more fair to make stat gains based on a RoT system like regular skills.
So my opinion: dont allow second characters. It would cut down on a lot more of the 'challenge' of Siege