Had John McCain been chosen as Bob Dole’s running mate in 1996, he would have been substantially better positioned to compete for the nomination in 2000. He might well have won, and had he done so he would have won the subsequent general election more decisively than Bush did.
Similarly, had John McCain lost with Tim Pawlenty as his running mate in 2008 (and he would have), Pawlenty would have had at least as good a shot as Mitt Romney of taking the nomination this time around, his uninspiring persona notwithstanding.
As things stand, Tim Pawlenty seems like he would make a perfectly adequate running mate for Mitt Romney to lose with to Barack Obama. Which would give Pawlenty at least an outside chance of being competitive in 2016 against Rick Perry and Jeb Bush.