Speed is a factor when mixed with abilities (dodges with speed are highly desirable). Sometimes you want an H/H for that 1 extra hit and sometimes a P/P for killing quickly. B/B isn't always undesirable (though usually is). To further complicate matters there are the hybrid mixes to consider, such as H/S and H/P.
The old stand-by of "it's not the size that counts, it's how you use it" applies to collecting breed types