I think the pump is important in the right context, for several reasons:
-Stimulates growth by triggering the secretion of various catalysts.
-Clearly indicates that the desired area's being stimulated.
-Psychologically important to training intensity for many.
However, a pump by itself is not sufficient; it matters how it happens. If a very light weight's used the result's fairly useless for size, but might develop capillaries related to slow-twitch endurance fiber. On the other hand, if a pump results from the use of at least moderate weight combined with intense training, IMO it's relevant to size and other attributes.
Conversely, low reps and heavy weights does not create much of a pump, and IMO it does not have the same muscle building benefit either, IMO.
It's subjective, there's no definitive answer yet. However, based on anecdotal experience it appears to me that it's part of the growth equation.