I'll put in my 2 cents here ...
While I understand the spirit of the rule, I do think it's pretty limiting. This hobby is based on the idea of collecting and a lot of the collecting that many people do is via buying single cards. This rule, while well intentioned, eliminates a large portion of cards that would otherwise be very fitting for this contest. Two scenarios come to mind and there are many more but here are the two I'd like to bring up:
1) Person A can't afford to buy a box of Museum Collection (or other crazy expensive product), or even a single pack for that matter but finds a PC item sold singly from that product which becomes the centerpiece of their PC.
2) Person B can't find/afford a pack of 1955/1960/1970/name any other vintage year cards but finds a rare Mantle that they then purchase.
These are just a couple scenarios. I think the spirit behind the entire hobby is to collect singles. At least that's what it was when I was younger and collecting. Back then, there wasn't as much of a focus on buying boxes upon boxes of product. We bought packs. We bought singles. We traded to complete sets. We traded three cards we loved for one card we adored.
It's the only rule I was iffy about when I read it but I figured I'd hold my tongue. Since it's still a discussion though, I thought I'd chime in with my opinion.