I have two concerns regarding having actual spoken dialog in the game.
Concern #1 is that the game is officially being localized in 5 different languages, each would require its own spoken dialog recorded for the game. Of course, this could be addressed by setting the stretch goal amount high enough to cover the costs of hiring voice talent and renting a recording studio, but I think the amount necessary to do so would ultimately put it out of reach.
Concern #2 is the fact Playtonic are offering the chance to voice a character in the game as a reward tier on their Kickstarter campaign. I don't know exactly how Playtonic are going to be handling that, but I imagine they'll be collecting samples of various vocal sounds from the people who backed at that tier that will ultimately be tweaked and tuned to fix any quality issues and to fit whatever character the resulting "voice" is assigned to. Something to that effect is much easier to do than having those backers record actual spoken dialog.