I think your BS detector has already gone off.
With any media, regardless of speed rating, a well-burned disc will last the longest. With high-speed drives, that usually means burning high-speed media at it's "optimal" speed, which produces the lowest error rate (C1/C2). With every combination of drive and media, there's a point where lowering the speed does not produce better quality discs. For me, that's usually at 40x or 32x depending on the media.
There are certainly a lot of people who are finding that their older CDRs, with lower speed ratings, have not stood the test of time.
It's also true that high-speed media is better made in the first place. But it's a moot point, because there are no media makers that are producing anything other than 48x rated media. Everything else is just old stock.
There are still lots of folks around chanting the "slower is better" mantra, mostly audio freaks. But this is slowly fading away as more and more discover that high speed works just great.
PS: I suspect your amiable clerk is just trying to unload that dusty old 24x media that's been sitting there for the last year.