Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-69.121.172.69-20130820040029/@comment-72.229.243.92-20140107151205

A purely in universe theory would be that the greater the transformation you make, the less control you hold over yourself. We all know that the werewolves aren't super good about keeping themselves in check. At best (Scott, Mama Hale) they display signs of arrogance, rage, jealousy. At worst (Erica, Peter, any of the Alpha Pack) they are narcissist, petty, obsessive, controlling (of others), untrusting, cowardly, with a tendancy to become complete sociopaths. We've also seen a tendency for these traits to be exacerbated whenever they're most wolf lie, ie. the full moon, sex, combat, etc.

Derek (born a wolf) always reminds Scott (bitten) in season 1 that his life will be a fight for control against these impulses, just as Derek's life has been.

Peter (born a wolf, killed his niece to become an Alpha) often fought in a fully transformed state, however he never displayed any of his usual human forethought or cleverness, instead ALWAYS relying on animal instinct and raw power to overcome foes.

What's my theory?

A werewolf good or bad, exists in a state of semi control over themselves at all times. If you are in your human form, you have the most control over yourself. However as you transform you willingly let your impulses have more and more free reign to the point that if you transform as far as you can, you are basically out of control (unless of course you are extremely powerful AND extremely strong willed, ie. Talia Hale, Deucalion).

So while it may be that you are more powerful the more you transform, few werewolves would choose to do so, since the lack of control would likely hinder them in most situations, including fights.

TLDR

Everyone can go fully wolf/bestial however next to no one has the necessary will to control this powerful form and so they choose to only partially transform.

PS I love Erica and would support any wild plot incoherency that brought her back, but being a wolf didn't do her any favors as a character.