I would not say that the new photo policy is useless. The minimum picture size was put in place to get the size of photos so that photos can be seen on all devices in use today.
Display technology has evolved a lot in the recent years with computer displays being larger and finer than 10 years ago and also smaller and finer displays on cell phones.
So a 300 pixel sized image would appear as a very small image on today's displays.
A seller may not have changed their computer display in the past 5 or 10 years but the world has changed a lot. What looks good on your display may not on a new display.
So eBay mandated a minimum size of 500 pixels to get rid of as many small images and photos as possible that were put in listing years ago when they were good enough.
Ironically, eBay did this to improve the eBay site when buyers were shopping and keep sales up. eaby cannot allow the site to get old and far behind in technology. So indirectly the minimum size is helping sellers keep up their sales or improve sales. Buyers with newer technologies will not see small poor photos and may buy with the bigger nicer photos.
eBay is actually helping sellers with smaller older photos stay up with technology changes they cannot see since by forcing them to upgrade their photos.
In the end these sellers will still be competitive with other sellers on eBay and other sites and eBay stays up with the newer technology for those that have it. So a win-win for everyone in the long run.
