mainly because I don't trust liquid yeast to be shipped during the hot months, nor do I have a local source for any. Not driving down to Brewstock in New Orleans just for yeast. Maybe I'll start using liquid once LA Homebrew gets up and running.
I only use Liquid yeast. I get it from Austin Homebrew, and never had a problem with utilization even during the hot summer months. However, i do make a starter for every brew, with the liquid yeast and add some yeast nutrient. I don't trust dry yeast to work right away, even when rehydrated.
My last starter for White Labs Irish Ale was so vigorous that my starter had a blowoff. And then 1.5 days after pitching, i had an epic blowff even with a blowoff tube attached. I lost 1/5 gallon of beer due to blowoff.
I prefer white labs, as well, for my starters, compared to wyeast.