I don't know about the AR-10 focus, but replace 'AR-10' with the name of any other evil scary baby-killing assault weapon for the purposes of this discussion.
While I fully expect the antis learned their lesson from the '94 AWB and won't make it so easy to circumvent this time around, if your objective is to amass "assault weapons" for posterity and not lucrative post-ban resale, I still think there's some merit to stocking up on lowers. Here's my thinking:
Assume President Obama's first official act is to urge congress to ban semi-automatic weapons. You're still looking at weeks or months of brutal political fighting, and possibly a delay in implementing the law. If before this all starts you're the proud owner of five or ten stripped lowers, you'll have time to acquire the parts you need.
Will there be a frenzy and run on parts and trouble finding them? Absolutely. But there's enough diversity in manufacturing for AR parts that you should be able to assemble an LPK, stock, and upper for each of your lowers. Will you pay a markup? Yes. Will you have to settle for parts that are available vs parts you wanted? yes.
Compare this to the guys who bought one or two complete rifles; assuming they have any cash left over, they now must scramble to get lowers as well, which due to GCA '68 aren't as easy to obtain. While you're mail-ordering uppers through the mail from some private seller in Texas (who, granted, is surely selling them at a markup), they're scrambling to arrange FFL transfers of whole weapons.
Generally I think buying guns because they might be banned later is a silly reason to buy guns, especially when there are so many other reasons (defense, fun, collecting, competition, did I mention fun?). This is especially true when it's not motivated by a specific political movement. But if you are to hoard weapons in anticipation of a ban, think through the particulars before you pull the trigger, so to speak.