The main caveat is that even if your plan allows you to make after tax contributions, you are allowed to make periodic distributions of these contributions (and their earnings). This allows your money to end up in your Roth IRA through a Roth rollover, where you have access to it if needed. Otherwise, it is trapped in your 401k plan until you separate from service or qualify for some other distributable event.
Funds in your Roth can be tapped for any purpose, and the 5 year holding period to avoid the 10% penalty only applies to the taxable portion of your conversion. If you contribute 20k and do the Roth rollover after a gain of $500 for example, the $500 will be taxed. If you need some of that money before 5 years, the $500 will incur the 10% tax, but the rest of the rollover was not taxable and will not be subject to the tax.
So before contributing, check what your distribution options are for this money. Finally, your total annual additions (Code 415(c)) are limited to 51k for 2013. That includes your pre tax deferrals, company match, forfeitures, and your after tax contribution amounts.
The principal benefit of course is that Roth IRA funds generate potentially tax free earnings, while saving in taxable accounts does not. But you also need the plan to allow periodic distributions to get those contributions into your Roth before they generate earnings in the plan to realize the main benefit. SInce you would distribute out your after tax contributions while still working, they would not be in the plan after you separate. If you separated while there was an after tax balance in the plan, then your entire plan balance is available for rollover and pro rate rules would apply to any distributions you took.