I don't know if it's still the default, but VB used to come with the ridiculous 'feature' of having the quick reply box 'disabled' until the logged in user clicked on a link/button to enable it for each post.
I believe there was an obscure 'logic' in there somewhere, but this was the first thing I dealt with when setting up a new forum.
History Lesson:
The Quick Reply needing to have a box checked on a post is a 'hang over' from the threaded BBS days. vB (at least up to 3.8) would allow the old style threaded view (if you turned it on in the Admincp). Forcing a user to check the 'Quick Reply' box on a specific post ensured the reply was linked to a specific post.
We old folks (Just call me Gramps ;) ) remember the days of threaded BBS software of the 1990's (such as the WebBBS script by Darryl C. Burgdorf). In fact, we *really* old people remember the threaded BBSs you had to access via telephone line modem (pre-public internet) in the 1980's such as Wildcat and BBS-PC. In fact, I ran BBS-PC on an Amiga, and served as a Cincinnati, OH fidonet node, from about 1986 to 1990... And working in the defense industry I was on the internet (ARPANET {DARPAnet in the defense industry}) before it "went public". It has been an interesting trip.
Few people use a threaded BBS these days, but they're still around here and there.
So - That's the 'logic' it came from. It may be 'obscure' to you, but not so very long ago in human years (as opposed to 'internet years') it wasn't ridiculous - Threaded view was how BBS software was written. Those of us with 300 and 1200 baud modems in the closet remember the start of the ember which has become the internet, and the evolution of the software many of us use today. These days the "Quote" option is quite similar in function (at least in vB where it links back to the quoted post) - It allows one to follow a 'trail', so to speak.
As to vB 4 - I still run 3.8 (no interest in vB 4 in part because my forums are very heavily modded and customized) on my forums so I don't know if they deep sixed that or not. I still have threaded mode turned on if a user should want to use that view (it can be interesting), and I have it set so they have to check the box in a post to use the 'Quick Reply'. That's the way I set vB up in 2001 (after changing over from what was UltimateBBS, and Darryl C. Burgdorf's WebBBS before that) and have never changed it.
If I was starting from scratch today I would make sure threaded mode was turned off as it is additional database storage and processing, not to mention it is old technology for all intents and purposes.
Edit Note: Now that I think of it, the software this forum runs on sorta reminds me of UltimateBBS... ;)