Slow zombies are not only boring, they are non-threatening.
How do you escape a massive horde of slow zombies? Walk at a brisk pace, or ride a bike. Ever seen the original Dawn of the Dead from the 70's? At one point they enter the mall, filled with zombies and they just run around them like they were pylons.
I never got how the slow zombies managed to "infect" the world. Its so easy to kill them or escape from them that it shouldn't have even been a challenge for any decently organized force.
Fast Zombies are at least scary in the "omg we have to run out asses off or we are zombie food" kinda way, ala "Rage Virus" or Dawn of the Dead remake, and CERTAINLY more threatening.