https://bugs.winehq.org/show_bug.cgi?id=39859
--- Comment #4 from Austin English austinenglish@gmail.com --- (In reply to Rosanne DiMesio from comment #2)
Re-downloading gecko and mono on every wineprefix creation is also a waste of bandwidth, and that's a very real concern for users subject to bandwidth caps from their ISP. I recently had to help a forum user who canceled the download for that very reason.
The installers are cached now in ~/.cache/wine and have been for quite some time, fyi.
I'd also point out that it's useful for systems with multiple users, as then the installers are only downloaded once instead of for each user. Also as Rosanne pointed out, it reduces potential for user error, and potentially allows for a more secure method of getting those addons (e.g., packages could be signed in addition to simply downloading over SSL).