The only down-side (assuming that the gov't doesn't revoke your ability to repay and then take the higher payments) I see is if you elect to take the reduced payment initially, and either can't afford to just bank that monthly check, or worse, spend it and then not be able to afford to repay it later, you've greatly reduced your long-term benefits. But I agree, it's a sound idea for those of us who can actually live within our means, and who will live a long time.
JJ