IMNSHO = In My Not So Humble Opinion. Which is the direct opposite of IMHO (In My Humble Opinion). To be humble or not ... that, sir, is the question! 'Cause I've never been accused of being humble by those that know me...
Anyway, back to the question - why have Goodreader if iAnnotate is so awesome.
It's more about Goodreader than iAnnotate. I don't need the smooth scrolling that you require, which is the only thing (IMNSHO -

), that iAnnotate is better over Goodreader. Goodreader annotates very well, provides for bookmarks and is a great PDF reader, if you want to do page turn by page turns.
Goodreader is also an excellent file cabinet, if I may be allowed to stretch an analogy. It is easy to download any file from Dropbox (even ones Goodreader can't read) and then use Goodreader to "Open In" in another app. For example, I have epub e-books; Goodreader cannot read them. But, I can download them into Goodreader, then open them in, say, iBooks. Plus, you can put files into subfolders all day long in Goodreader, directly connect it to a PC (that has iTunes), use it to play movies or MP3s, etc. It's just very versatile and it's an app that I use almost daily.
Given my requirements, I'd pick Goodreader any day over iAnnotate, because it can do so much more. Yet, you want a smooth scrolling for PDF files, similar to Dropbox's previewer. Goodreader can't give that to you. In the interest of full disclosure, I bought iAnnotate before I bought Goodreader. I still use it, just not as much as Goodreader. And I cannot tell you ('cause I haven't used it enough) that iAnnotate isn't just as effective at the things I said Goodreader can do. I'm just happier using Goodreader.
So, if you give up the smooth scrolling, I'd recommend Goodreader at $4.99 (USD) in a heartbeat. Else, as far as the apps I know of - it's iAnnotate at $9.99 (USD).
Actually, I'd rather have a reader that does a proper re-flow of PDF files (like my Pocketbook 360 e-ink reader), but so far, nothing. But, that's a tale for another day.
Hope this answers your question(s). Got more - let me know.
Marilyn
P.S. I also have neu.annotate. Yeah, it's a very nice app, but it does not have the smooth scrolling you want... I also have ReaddleDocs. It does vertical scrolling, but it's page by page, not continuous scrolling. Yeah, I like reader/word processing apps. Can you tell?