Are inner scrollbars a bad ux?

Looking at the gmail interface when replying a message, I can see the original message and just below it, the new message window. this pattern creates two vertical scrollbars: one in the browser window, and a second one in the inner 'new message' window. The same pattern exists here, on ux.stackexchage, when writing a new question and observing the 'Questions that may already have your answer' section just above it.

I can't seem to find many advantages of using this pattern, whereas the disadvantages are clear: small area for the inner scroll, strange mouse wheel behavior on the page etc.
From my point of view, it just seems as bad ux pattern. Can someone enlighten my eyes and give me some examples of places where an inner scrollbar pattern gives a better experience to the users?