Why is it rare to combine Reformed/Calvinist doctrine and Dispensationalism? I've been told that it's rare to find someone who combines Dispensationalism and Reformed/Calvinistic doctrine (such as John MacArthur).  Why is this?  What ideas don't mesh well?
The reason these two are seldom found together seems to be primarily because of the difference of the view of the church between covenant theology and dispensationalism. Although some (like MacArthur) continue to maintain the distinction between the church and Israel (as in dispensationalism), most 5 point Calvinists follow covenant theology which denies this distinction - placing the beginning of the church at least back to Abraham, and logically back to Adam. Therefore, although dispensationalism maintains that everyone has always been saved by grace through faith, the specific object of that faith in Reformed theology is understood to be Christ - while dispensationalism would recognize the role of the progress of revelation such that the basis for salvation was the Cross, the object was not as clearly seen as the incarnate Son of God. Actually, it is even more complex than this, but this gets you going in the right direction.