It doesn't matter that you don't really have to buy WinRar/WinZip to use them. Doing it *is* technically illegal to use both WinZip and WinRar without the lisence after the expiration date is gone by the wind... even if it's not actually enforced in seemingly any other way than having to wait a couple of seconds and then clicking "OK"...
But anyway. If you want maximum compatibility, use basic *.zip compression. If, however, you use *.rar (which is done a lot anyway these days), you effectively do "force some other program down their throats" so they can open the file, since they will need some tool to open the rar archive... and because you have to assume they have to download some program anyway, it's best to point them to 7zip directly.
Assumption that they have WinZip or WinRar installed is actually just as valid as assuming that they have 7zip installed already. Granted, they are currently more widespread due to their longer exposure to public, but 7zip is spreading fast, because it's both free
and legal to use without registration fees, and it doesn't nag about anything either. And makes smaller archives to boot.
Also, the program install file is a whopping 818 KB (my current version). With a link to 7zip pages, it's not really too much to ask from downloader to go there, grab the file and install it. In exchange of this small task, the file size can decrease substantially which reduces upload time, download time, bandwidth usage and the cost of the transfer to the hosting server's owner.
I really have no problem with archives of multiple formats - as said, purpose should define the archive type used, but in general isn't smaller file a preferable choice?
