Pure speculation (stating that up front):
I can imagine that there may/might/possibly come a point where certain plugins are only "Logic X" compatible, or only Snow Leopard/Logic X compatible. And if it turns out that Logic X ain't all it's cracked up to be then anyone running a (ahem) "legacy" system won't be able to use it. Now... will that be the end of the world? Unlikely...
For perspective, there are lots of music professionals who do not subscribe to the idea of updating for the sake of it. There are plenty of older systems in use, and no one's the worse for wear or producing "worse" music than people running the latest/greatest. Certainly, updating is sometimes highly beneficial. Were it not for the update from Logic 8 to Logic 9, Logic users wouldn't have a Flex feature, amongst other things.
But the idea of updating or upgrading just because Apple comes out with something new makes no sense to me. Every time you change a stable system, whether it's to update/upgrade the OS or an application, you run the risk of destabilizing something. Look at Logic 9.1.4... new bugs have been introduced that weren't present in 9.1.3. So every update or upgrade carries with it a risk, whether it's bugs, breakage of functions that used to work fine, or a change in workflow. And who wants to retool constantly? In the case of a feared Logic X ~ FCPX (the "~" meaning "similar to") the problems you'll bring upon yourself will be far worse than software breakage.
But again, this is all total speculation. No one knows what the future's going to bring. When Logic X shows up it will be what it will be. It then becomes your choice to update or not. Think of it this way. Today, Apple releases Logic X. Now what? Does your ability to make music suddenly stop or change? No. You're simply presented with a new option, and as a free-thinking human being you can take it or leave it.
This editorial and unabashed opinion (with a touch of Zen) has been brought to you by Ski, purveyor of fine musicks on a variety of cutting edge as well as legacy computer systems since 1987.