For my sanity, I've reasoned it this way.
"I'd like a Coke."
"Diet, Vanilla, Cherry, Cherry Vanilla, Orange Vanilla...."
"OH GOD, please just a Coke Classic!"
Binomial nomenclature helps to always ensure two separate, yet related, "species" are well-defined.
As I'm looking for support for 10.5 Leopard online, I run into the battle of filtering out the added "Snow" that inevitably appears in the results. Same if, for instance, you want support for an original PlayStation...but you can't filter out PS2 or PS3 etc. easily.
If, in a parallel world, Apple called 10.5 "Panthera Pardus" and 10.6 "Pantherea Uncia", there would be NO confusion, NO difficulty with filtering and far fewer bad leads.
If you called one a MacBook and the other a MacBook Pro, you'd have a hard time defining what makes one not like the other. Remember the 2008 13" MacBook? What made it not a pro? Nothing, so they changed the name.
So if you walk into an Apple store and ask for notebook options, they will present the "air" or the "pro". When you ask for clarification, it's simple: air means light, portable, great to use on the go. Pro means the most power and features for professionals and dedicated applications.
I was pretty sick when they called the 12" a "MacBook" even though it was smaller, lighter, and less powerful/feature packed as the MBA. That is when they should have swapped tags. But, since modern notebooks have whittled out the need for three "tiers", I like the current naming...so long as MacBook doesn't show up beneath the Air again!