Hi James,
I just came out from a conference call with SAP with exactly the same question. I didn't get a good reply from them. I am glad, I found this. I am feeling good after reading this post. We are also currently planning to activate catch weight management and we are worried we can't use IS functionalities in future. Looks like it is not a problem. Please let me know, if we can talk to you to get some more inputs.
Regards,
Srini