I wouldn't say it's pointless, it's just about flexibility really. But I get what you mean. If an MX is already heavily loaded or in need of upgrade, it's about giving an option with less performance impact (less impact on the MX itself and less impact on the cloud lookup service). And I wouldn't say top sites is doing a SMALL number of just the most common sites, it typically blocks the good majority of the common sites for a given category. Naturally the less popular one-off sites will be allowed through, but it's less common, and of course you've got blacklisting, and don't forget you can turn on Syslog and capture every URL being hit. It's definitely a "your mileage will vary" type of thing.
Full list is obviously more intensive, and if there's lots of categories turned on, along with other advanced security features, there's more of a performance hit. Top sites works pretty well in many cases, but of course it depends on the deployment and user base, and if there are any legal or liability concerns. And in that case, size the MX appropriately and go Full List. While it may be a bit more cumbersome, I do have many customers who leverage top sites plus blacklisting and URL syslogging.
Also consider, this feature has been around from the very beginning, and I'm not saying it's deprecated, but several years ago it was a much bigger deal than it is today forcing so many full-list URL look-ups to the cloud over slower ISP links. Likely not as big a deal now as it might have been a few years ago.
