The only thing with being able to move just one Zombie, it might make it a little too easy to thin the herd (as you can attract one Zombie and then kill it. I thought the point of it being 2 is to avoid this? (but this was only my interpretation).
Theme wise I felt that it's a little weird that you can call over a specific number of zombies anyway!
Technically, if you have certain survivors or weapons, you can do the same with 2 zombies too.