You can integrate other amp simulators into a chain with Guitar Rig in it while replacing the GR4 amp cabinets with impulses. I personally recommend Lepou plugins since they're both free and awesome.
This is the chain I use for my distorted tone. I run Guitar Rig 4 in front.
Personally, I find GR4 to be incrediby useful for cleans (I love the Fender style amp), effects and even as a bass amp