By "Pure AutoLISP" (let's call it vanilla) do you mean entget, entmod etc.?
My opinion: for no real reason you're denying yourself the benefits of functionality not offered in vanilla lisp, or methods that are more convenient | efficient.
I was relatively late to start using extendo lisp because our company as a whole lags behind the leading edge (if we start a project with version x we finish the project with version x; many projects span 2 or 3 years). Having used extendo lisp now for a couple years I wouldn't go back.
Having said that some times vanilla lisp techniques are still required, so I don't think you will ever see any medium | large lisp applications that are pure vanilla or pure extendo.
Anyway, take it or leave it, just sharing my thoughts on the topic.
Have a good day.
Cheers.
Edit: I see you posting vla-* code in other threads so ignore the evangelical bits above.