For brokers that allow you to buy bonds, its very natural to buy it and use it as collateral. The tricky part is when you have a broker that does not allow bond buying and perhaps even futures buying (I have a couple like these). It feels unnatural to go long something in another account, in order to 'earn' interest in the first account but its the right thing to do. I never read this sort of advice before perhaps because its a bit unusual but i think its right. its all one big account called 'your networth' anyway.
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.