Yes, it is worth investing post-tax dollars in your 401k
if you are not eligible for a ROTH. The money will grow
tax deferred. You can always withdraw the amount
you invested, with no tax penalty, at the time you
do a roll-over IRA. Just make sure your company
keeps track of your post-tax contributions. That is
why I suggested that you use different funds for the
post-tax investment ..... just belt and suspenders.
OTOH, if you invest in a tax efficient index fund like
Vanguard's Total Stock Market, you will get almost
all the tax deferral benefit with less potential hassle.