Log rotation isn't just about disk space (although why you'd want to waste Gigs of disk space on storing old log messages locally is beyond me), it's also about helpful to segment logs for easier and quicker searching and reporting, either manually or with scripts.
Not to mention it's pretty much automatic these days. It's not like it's something you've got to go out of your way to configure.
I disagree, since, otherwise, there's nothing rotary. If one has effectively infinite disk space, nothing ever gets "rotated out," for example.
Certainly, log segmentation provides benefits beyond being a prerequisite to rotation, but, as you point out, it's a common default configuration. An inbuilt feature of syslog-ng is log file naming based on date, obviating any post-processing.
Not to mention it's pretty much automatic these days. It's not like it's something you've got to go out of your way to configure.