Do you memorize the IP address of every website you visit? Do you even know the IP address of the mailservers that your friends use?
I heavily doubt it... Like the rest of today's user community you probably rely on DHCP to get your IP address, probably without even knowing what it is, and use DNS so you can direct your browser to
http://forum.feed-the-beast.com instead of having to type in the IP address every time.
Using numeric IDs is prehistoric in comparison to the new system, much like the way we had to manually edit our systems' "hosts" file to include sites we wanted to visit without having to remember the IP address before DNS was invented, or remember the address of every site we wanted to visit before that.
Before DHCP came along you had to pay a pretty hefty monthly charge just to get an IP address assigned to you, or to get access to a system with a pool of loanable addresses, since the people/companies that owned the subnets had to pay for them as well.
The point is that doing things the way you suggest adds more background processing the larger the number is, slowing down the game, does nothing to solve blockID conflicts and would have to be increased again in the future as newer mods are written and their IDs have to be tacked on to the end of the existing list.
This way, every client has it's own table of available IDs that is loaded with nameIDs in the order they are registered, kind of like a DNS routing table, so you don't have to have a static ID table with every ID used by every mod in existence on every copy of the game for things to work right. If a mod is not used it's nameIDs aren't even loaded, which leaves more IDs for the mods that are loaded to use.
I read, probably on the Curse Minecraft forums, that the maximum number of IDs has been increased as part of the new system as well.