Well, a few things here. Say you have broker XYZ and they don't allow access to cash treasuries. You can buy the treasuries direct and transfer them to broker XYZ to fund your account. Serves the same purpose. You don't need to be able to trade them because you are using them as a fixed income investment. Besides they are highly illiquid once you get them. BTW, the other benefit to this appeared when MF Global blew up. Anyone who kept their money in their futures account as cash lost it. However if you bought treasuries and kept them at the FCM, your money was protected. The treasuries are in YOUR name and cannot be liquidated to meet the obligations of the FCM.
Buying and hold a 10y bond to maturity is very similar to rolling a 5y future over and over again. The 10y has more duration risk at first but since that declines over time (as it gets closer to maturity) the average duration will be similar to the 5y future. Point is its a very low risk strategy, I havent looked at the numbers but I suspect it makes money at year end like 80%+ of the time. If the Fed goes 1994, sure you will lose but as I said, that is offset by the fact that you now get to invest at higher rates and hence, that makes it less likely you will lose going forward
Also, the risk that you take buying the futures has to be weighted against the risk of doing nothing. Doing nothing costs 1.5-2% per year (inflation), so if doing something on average will help offset that. its a decent idea