The Blend If settings are extremely helpful tools. Knowledgable use of the Blend If sliders is also a technique that distinguishes the intermediate Photoshop user from the novice. I discussed the Blend If sliders in my tutorial on blending two images: Blended Exposures Revisited. In addition to blending photographs, the Blend If sliders are critical to effective sharpening of photographs, where they are used to protect the extreme shadows and highlights from being clipped. They are useful anytime you want to limit the effects of an edit to pixels based on brightness.
This reason that Blend If sliders are not used more often is that most of the discussions by Photoshop gurus are confusing. Even fine technical writers like Bruce Fraser and David Blattner make a confusing mess out of the explanation with talk of things like eggs and baskets. This brief video applies some plain, photographer-to-photographer talk about the Blend If sliders from the perspective of controlling where sharpening effects get applied.