There is some confusion here. To be perfectly clear - T1000 is the toughest, strongest fiber you can buy today. It is not 'more brittle' than any other fiber, it is stronger, tougher than any other fiber!
There is a new fiber (along with with a corresponding resin system) coming from Toray, T1100, that will be/is even stronger but it's not widely available yet.http://www.toray.com/news/carbon/nr140310.html
So no, limiting the use of T1000 from a strength perspective makes absolutely no sense!
Limiting it from an economic perspective makes absolute and total sense! Again just look at the chart above...
Yes UHM fibers are more brittle but T1000 is not a UHM fiber! It is, like I pointed out above, a high strength fiber.
BUT you need to match the resin system to the fiber! Or more specifically every fiber brand/type has specific surface properties that you need to match the resin system to.
There has been some problems in the aviation industry reported with the toughness of both T800 and T1000 prepreg but that had to do with the matching of the resin system.
Correct matching of the resin system is of paramount importance.
There are resin systems that will create a stiffer tube with a given fiber but it will be less tough than a less stiff but tougher resin.
You have to look at the combined performance of the resin-fiber system.
Edited w link to Toray
Edit - the new Mitsubishi Pyrofil MR70 intermediate modulus fiber claims a strength over 7GPa and would then be beating the T1100G (dream carbon in the Dogma F8) for stiffness.