I think as important as leucine appears to be for muscle building stimulus, the other BCAA constituents (valine, and iso-leucine) have their importance too.
Here's a study published in 2010 that focuses specifically on iso-leucine and how it might even be better than leucine for fat burning: http://www.ncbi.nlm.nih.gov/pubmed/20089773.
Leucine in itself has proven thus far to be a very important amino when it comes to anabolism...but life in general is not solely dependent on anabolism. I fear sometimes in pursuit of things that are predominantly muscle building stimulating that often "balance" with other important nutrients gets lost.
For muscle building, leucine is perhaps the most imporant amino....but for muscle building, the body needs all the proper enzymes, hormones, etc built from OTHER AMINOS to support it.
In my mind, ensuring that the approximate 3.5 grams of leucine per meal is adhered to for maximal muscle building (combo of leucine content in meal itself plus any additional supplementation) but also making sure the rest of the diet contains enough OTHER nutrients is of primary importance.
Balance. Something that needs to be remembered in everything. Without it, something somewhere sometime is going to give.