I dunno. I'm not a big fan of relativistic time because it would mean the Earth that the Enterprise crew knew wouldn't be the way they remembered it when they get back. They would be even more out of sync with time than Captain John Christopher being brought forward into the future. I figure if they have artificial gravity then time dilation is something that they could have solved as well.
Well, exactly. That would be one reason, beyond just being couped up on the ship for subjective years, why they'd want to recharge the warp drive. It's only something that they would endure in an emergency.