Why? Well, I’m almost certain that it messes up the readability of longer texts.
I came across a couple of websites recently that use the following CSS bits on dark body text
text-shadow: 0 1px 0 #fff;
or some such thing. Of course, sometimes including the vendor-specific extensions as well.
Please, don’t that. I pretty sure that it makes body text harder to read and I’m not quite sure whether it really looks that good. I mean, I haven’t got any scientific studies at hand and it somehow looks good in the beginning but once you try to read it, your eyes are complaining sooner or later. At least my eyes are.
I will conduct a non-representative study about this but until then: Please only use this effect for headlines. Please!