Before LR/CR4.1, I found it sometimes worthwhile converting an image to L*a*b for enhancing colour separation and saturation. It is also correct that the Blend IF sliders worked on the *a* and *b* channels in Layer Styles help very much to target and control the impacts. (But for luminance work, as observed above, one is just as well off using the RGB composite curve in Luminosity Blend Mode). At the same time, it must be said, this L*a*b is not an easy space to work with - takes alot of experience learning to control it and to predict what various curve moves will do, because it is anything but intuitive. It is also inconvenient making trips back and forth between colour spaces because Adjustment Layers cannot be preserved, so the workflow needs to be organized around trips to Lab in a way that avoids that inconvenience. (Here I'm ignoring the loss of information associated with colour space exchanges because that has already been well demonstrated and thoroughly discussed.) Hence all these very obvious and practical factors taken into account, if there are easier, non-destructive ways of achieving the same results without exchanging colour spaces, so much the better. LR and CR4.x have vastly increased this capability, not only because of the Vibrance and Clarity controls in the Basic Tab, but also because of the 24 independent targeted controls one has in the HSL Tab for eight colour groups of significant interest to photographers. To sum-up a short answer to the OP's question: do as much as the new (and old) tools allow in LR/CR4.x and CS3 before moving to L*a*b, and if you still find you need a particular effect just not otherwise achievable, go ahead and try L*a*b, but be mindful of the workflow implications.