Because simply replacing the seals is usually all the head needs. I proved this to myself after replacing the seals on my 545i. New seals solved the oil consumption issue and the plugged secondary air port issue (not applicable to newer V8).

With AGA's tool, the seals can be replaced without removing the heads. Time and money saved. And a DIY task. The smarter choice in my mind. I,suppose it is possible that excessive guide wear could be an issue; but, not the case on my car with 125k at time of seal replacement. BMW provides a spec on determining allowable guide wear by measuring side to side movement at the top of the valve stem.