If ObamaCare is delayed, it dies
Rubio's proposal seems like smart politics, given that there is no way the Democrats can accept it. If there's going to be an individual mandate delay, Obama will do it unilaterally, and it will be presented to us as the benevolent majesty of the King, rather than a concession to the accuracy of Republican criticism.
There is no way Democrats can survive accepting the same sort of delay proposal they shut down the government to crush. It'll be hard enough on them if Obama issues an executive order, but at least they can pretend he's exercising some sort of emergency measure that he wisely built into the Affordable Care Act for just such a contingency. (In truth, he'll be stretching the hell out of a provision that allows "hardship exemptions" from the individual mandate, essentially classifying his craptastic website - Jon Stewart just called it a "turd" - as a hardship that affects everyone.)
But I doubt Obama will grant that exemption unless he's completely backed up against the wall, because it would exacerbate the death spiral already facing ObamaCare providers. Young suckers must be forced into the system to pay huge premiums for those high-deductible plans they'll never actually use. It's debatable how much the threat of the individual mandate intimidates them - it's a lot cheaper than the nearly-useless coverage they'll find waiting for them, if they get past all the error messages. But it almost certainly does have some effect. People are generally inclined to avoid fines, even if they're relatively mild.
Also, since the entire point of this misbegotten exercise was to force all the "free riders" into the insurance market, formally rescinding the key conceptual requirement is a pretty rough intellectual concession. The insurance companies certainly won't like it, especially on the timeline Rubio is proposing. Six consecutive months of smooth exchange operation? The way things are going, that might bring the individual mandate hammer down right in the middle of the 2014 elections. (Or maybe even after them, if you subscribe to the gloomier estimates of just how badly FUBAR the exchange system is.)
There are many other provisions of ObamaCare that would face practical or legal jeopardy if the individual mandate was delayed, so whether it's done by Congress or the executive, it would greatly strengthen the repeal movement. But most importantly, we should remember that ObamaCare is a mousetrap, and those don't work very well if they snap shut in slow motion.
This whole scheme survived this long on a tidal wave of lies. Three weeks after launch, they're gone. Not even the Obama dead-enders are willing to claim you can keep your health insurance or doctor if you like them any more, or that most people are going to see their premiums reduced. The bill would never even have gotten past the Reid-Pelosi Congress if Obama had been honest about those matters.
The clock is now ObamaCare's enemy. Delay part of it, and it may swiftly become necessary to delay all of it. A respite from part of the bill will give angry Americans a chance to scuttle the whole thing, and that's a risk Democrats cannot allow. Especially not when they're up against people like Ted Cruz, who have been consistent in their opposition - and honest about the duty of responsible representatives to oppose this train wreck with every tool at their disposal.