Additionally, 10's trade in rate while MBS trade in price, with mortgage rates being a less consistent extrapolation of MBS prices based on a number of factors. Consumers and mortgage professionals alike can much more easily understand a move in rates than a move in MBS prices. Treasury rates move in real-time while we're beholden to lender reprices when it comes to seeing how much mortgage rates will move.
Treasuries also trade overnight, all around the world. They also trade in the futures market, offering a level of liquidity and price discovery that MBS can only dream about, and in that futures market 10's in particular, are the most active issuance of any maturity remotely close to the average MBS maturity.
Treasuries express a purer form of the bond market psyche. To whatever extent "risk-free" exists, they are that, while MBS are only virtually risk-free (keep in mind, that's from a default standpoint where Fannie and Freddie guarantee payment of principal and interest. In other words, foreclosures and the like have nothing to do with default risk for MBS). Treasuries also don't have to worry about changing consumer behavior based on interest rates or other considerations. Such things affect how quickly MBS get paid off (refinances, sales, foreclosures, etc), which has a direct bearing on the value of a particular MBS pool.
In short, if there's one great place to look when it comes to examining relevant trends to MBS, it's 10yr Treasuries. They're a large-scale, super duper 'vanilla' poster-child for the broader interest rate movements that matter to MBS and mortgage rates.
They are the backdrop to MBS's brushstrokes
They are the sea to MBS's dinghy
They are the weather to MBS's clothing choices
They are the black lines to MBS's colored pencils
They are the bumper bowling lane to MBS's 16lb balls
Paint can end up flung far from the canvas,
Dinghies can sink or be carted off to the scrap yard,
Folks can choose to wear shorts in a snow storm,
Kids can spend more time coloring on the desk than inside the lines,
and as you've no doubt witnessed at least once, under the right circumstances, a properly flung bowling ball can show complete disregard for the bumpers.
In other words, there's always the risk of an exception to the natural relationship between these things, just as the natural (or "average" if you prefer) relationship between MSB and Treasuries can vary a little (day-to-day) or a lot (2008 or QE3 Announcement). In those cases, MBS Live will ALWAYS make note of relevant departures from the norm between MBS and Treasuries. When such departures are absent, it will ALWAYS make sense to use Treasuries for the longer-term overview of interest rate trends.