I read in other posts of the link above that it´s limited because the save-games need so much space. I´d call this a design flaw... I dont see why a save-game on theyr server cant just contain text, in that case it wouldnt need much space and could also be zipped.
WoW had 6 slots from the start when I recall that correct. It was increased with every addon.
I still find it cheap because I can actually play 4 classes, engineer, science, tactical (who I got already) and klingon... ergo there should be at least 4 slots available.
You don't get a whole bridge crew on WOW either. Each of those Boff's needs space too.