I can totally picture myself buying more character slots. It would cut down on the time I spend switching servers and would -for me- fall into the convenience category. Also, I think character slots are priced just right in the Store right now, so the TP tag on them wouldn't stop me from buying if more were available.
Currently, players can purchase up to 10 character slots in the Store. I think a good number to raise that to would be 20 or 25.
Another aspect to take into account by those who'd like to have character slot for every race-class-combination is that the slots from the Store get added to whatever other slots a player has. So the 2/3/5 base character slots of the appropiate account type and the 2 character slots from the Moria expansion would also count against the 25/45 character maximum for completionists. If things were to be set up so that a total of 25 character slots could be reached without an active subscription (which I think should be the case), that would mean that a player would only have to purchase 23/22/20 character slots from the Store without Moria and 21/20/18 with the Moria expansion.
And as 25*2=50>45, players who also want to set up all possible gender variations could do so by utilizing just two different servers.
German, female, migrated from CodeMasters in 2011, Premium member, slow-leveller, pure PvEer and opposed to Forced Emotes - wherever I fit into the statistic, it's not in its middle.