Actually, I don't think it is necessary that decode_call take the flag, because if you don't pass in any Val objects, then everything will be an int anyway. It currently isn't possible to encode boolean unless you use Val, so if you need to, you will need to use the new features.
Sounds like you're talking about encode_call, and you're right in that case. However, decode_call takes an xml string and returns a Call object, so it could also benefit from the new decoding feature. I believe decode_call is intended for incoming calls when one implements an xmlrpc server.