Steve has a point - Breeds are often rare for a good reason - And most breeds I can think of their downsides - Some are escape artists, others too small, bad grades, slow growing, too much wool, strike, feet etc, and Most farmers have stuck with simpler systems based around breeds perfected to their area, and that simply means a lot get crowded out as they dont match, that said they do widen the gene base and are very valugble in that respect.
Something as simple as the fact that using Hebrideans to cross with texels to produce a breeding ewe - the resulting 1st cross is an excelent animal, grading well in its own right, fast growing but alot of the hardiness of the hebridean, and the ability to thrive and grow well on VERY poor grazing, and live out all year - the genetics are coming from the rare breed, and have a place, but on its own the Heb is difficult to make pay in the purest sense.