Whenever Rank 9 + comes out,we will be introduce with new skills.There is some build right now requires more than 4 skill bars.I can imagine when the rank 9+ comes out we would need more slots to put the in the new skills.
Perhaps sell 1 skill bar for a max 6 skill bars per character for a TP price so you guys can benefit it.
Also with the addon you can manage your skill bars the way you want and move them.Like for example have them vertical at the edge of the screen instead of horizontal.