To me, the two represent two different things:
vIta'pu'be' - "I have not done it, ever (in the scope of time being considered)"
vIta'be'pu' - "there has been at least one instance of me *not* doing (in the scope of time being considered), but I'm not necessarily denying that there were also instances of me doing it"
For example:
qaStaHvIS DIS vorgh, jIroppu'be'. - "Last year, I wasn't ever sick."
qaStaHvIS DIS vorgh, jIropbe'pu'. - "Last year, I wasn't always sick."
The distinction becomes a bit clearer with -ta':
qaStaHvIS wa'Hu' ram Hoch, jIQongta'be'. - "I didn't manage to get any sleep last night."
qaStaHvIS wa'Hu' ram Hoch, jIQongbe'ta'. - "I was able to remain awake last night."
//loghaD