In my experience, any benefit you get from having light substrate will be outweighed by a few other factors.
First off, if you have a very light substrate then you'll be able to tell VERY quickly when it gets a bit of algae, mulm, or fish poo that is common in a planted tank. It will get gunky looking very
quickly and won't look too hot.
Secondly, I have noticed that fish, when given a darker environment as far as backgrounds and substrates, tend to hold better color. A white background or white substrate washes out the colors of the fish dramatically. You may be able to avoid this if you have a lot of plant cover, but it is something to keep in mind.
Thirdly, a white substrate is very unforgiving when it comes to evenness and layout. It silloettes your foreground and draws attention to any flaws in the "horizon" created by the surface of the substrate.
Check out http://showcase.aquatic-gardeners.org/2006.cgi
You'll see that many of the BEST tanks go with darker or more natural colored substrates, mostly for the reasons I've outlined.
Anyone else have opinions?