User blog comment:Jimeee/Dragonborn DLC trailer released./@comment-76.254.54.122-20121108191418/@comment-3492791-20121109081917

As pointed out by Darthorc on another blog, that would actually create a massive level gap, being that many higher level players have like 100+ unused souls. If the system required the use of Dragon souls, they could just buy every single perk instantly, breaking the balance and purpose of the tree.