It grows, just not as well. But it still grows.Okay, if I get a free match for a poor performing fund, why not just invest in a better fund? I'd only give up $3500/year max in match, and if the fund underperformed popular funds like VFIAX, what did I gain myself except pretax deduction, which again may or may not be worth it, depending. That is what my hangup is.
Then when you change employers you gain the potential of rolling into a better fund.
For all I know, you might want to look into rolling into a Roth once vested--there's a tax implication, but if you are convinced that it's under-performing that much, you might be able to go that route.
In the end, it's free money. Giving up a 5% match is like working 2 weeks a year for free.