Do not forget there are independents beyond the repubs and democrats. If maximum dems (39%) and maximum repubs (37%) turn out, there is still 24% still to vote. Those are the block of voters who always decide who wins.
What you say is true but:
1) historically the independents have broken for the challenger
2) across the swing states it looks like large numbers of these independents have actually changed their registration from Democrat to Independent since 2008 (and not from Republican to Independent). Kind of shows you about the way they lean politically.
3) poll after poll show Romney having a lead with independents