Board Thread:Versus Debates/@comment-30830650-20170116074632/@comment-9782468-20170121055735

@Lorenzo, unless a Shinobi actively moulds chakra, their chakra doesn't replenish fast enough to maintain in a fight. If that were the case, they'd be able to fight indefinitely. We know that isn't true because even those with the largest reserves (Hashi, etc) could only manage a hard fight for 24 hours at max.

Basically, Sage mode give your more quantity and quality chakra. But it's finite; in a fight, you won't be able to replenish your regular reserves fast enough to just keep renewing Sage Mode.