Well, people at the craft beer industry just doesn't have limits, so, guess what? They are doing primary fermentation with Brettanomyces for many styles, including popular ones, like IPAs (look for brett ipa on Google). I don't know very specific details about the strains used for it, but I do know there are a few ones in the market right know. Probably some of them are more suitable for it than other ones.
As far as I know, Brett doesn't take longer than any ale yeast to ferment (primarily), and gives a slightly different aroma and taste profile, but nothing funky like it does in secondary fermentation/maturation, which is that slow process you probably have heard of, when it eats complex sugars and other compounds, ending up with that barnyard aroma. And contrary to what many people think, it doesn't acidify the beer (brett produces only a small amount of acid, not enough to sour it). When people want to brew sours, they use bacteria.
Actually, at a glance, 100% brett beers aren't so different from beers fermented with Saccharomyces. Off course, this is completely different from adding brett to the bootle and letting it to develop over time.
Why brewers are doing it? A guess they ask the exact opposite: you should we not do this? =)