Lots of reasons to use multiple strains. Flavor profile, attenuation, any reason you might want the best of one yeast and balance out the negatives of another.
I just finished a batch of pumpkin ale using British Ale. It is HIGHLY flocculant, but a little more attenuative than WLP002. The beer tastes very good, even still flat from the fermenter. I may replace WLP002 with this yeast. It was finished and clear in 10 days.
I haven't used East Coast ale. From the white labs page: its less attenuative than CalAle, and even less flocculant. Its listed as having a similar flavor profile.
Based on the above, I don't really know why I would blend these two yeasts. I don't really see how East Coast compliments '008. Besides, I don't like low flocculant yeasts other than heffe.
