Sure.
When Valve developed ssbumps, they combined ambient occlusion element, aka the "natural" shadowing that occurs when surfaces that face each other are near (read
this if you're unfamiliar), with the normal data that's contained within the normal map for a given texture. Ambient occlusion is really handy in game art because it implies 3-dimensionality and a "sense of proximity" among the elements within a texture, but when painted into the albedo, it's totally static. The reason why it's contained within the normal map instead of the albedo/diffuse is that, with self-shadowing normal maps, the ambient occlusion is meant to be
directional, and disappear when it's directly illuminated, creating a more dramatic and more accurate result. This means that if you paint any actual light into the albedo, it essentially eliminates the effect the ssbump is trying to achieve.
Here's a shot of one of L4D's brick textures, as seen in Hammer. Hammer doesn't render normal maps in its 3D viewports, so what you're looking at is purely the albedo:
See how flat-looking it is? That effect is totally eliminated in-game, when the AO of the ssbump starts working its magic:
I lit the wall from below to show you how much more natural the result is when using ssbumps on textures that are lit from bizarre or unusual angles. Because no light/AO is in the albedo, you get both directional highlights AND directional AO, and lighting it from
any angle will still produce an accurate result.
Fire up Hammer for Half-Life 2 some time and look at the brick textures, and then compare them to some found in Left 4 Dead; you'll quickly spot the difference. I hope I haven't confused you any more than you already are.