Okay, so the first question is... why don't you like autotabbing? Answering that will give you your case against applying it.
Autotabbing is an unusual behaviour on the web, and it doesn't work well unless users will only ever submit data to a field of a particular length and format. Applying autotabbing to specific tabs only is going to mean unexpected and inconsistent behaviour unless there's always a clear rationale to the user, or if your users are already exposed to the concept through tools they already use in the domain. A user who incorrectly expects autotabbing is going to stumble, which erodes their confidence in their ability to grok the way your application works, and a user who doesn't expect it is going to be similarly surprised - and if other messages appear when the new field takes focus, I would expect users to assume that they'd accidentally triggered an event of some kind.
So that means that autotabbing is going to have to be applied throughout at least a particular class of forms, and that's going to apply a rather strict constraint. Constraints are kryptonite to software development because, as we all know, requirements change, and the reasonable constraint of yesterday becomes a point of crisis tomorrow.
That being said, if your biggest issue with autotabbing is that you can't update existing software, you have far bigger problems than UX. Applications should encapsulate their view logic well enough to stop this being a problem. Then again, it sounds like you've inherited some legacy software, so the horse may have already bolted on this one.