You NEVER know what you're going get.
Barry Goldwater was once a strong conservative and in his later years wondered to the left on many issues he had previously been to the right on. At one time Hilary Clinton was a "Goldwater Girl" and we know how she turned out.
No one, not even Ann Coulter, can name a single prospect for the Supreme Court that we KNOW will remain consistent.
You can be darn sure what you're going to get from a Democrat appointee. Why not from a Republican?