I've always thought of the dual mode clients (e.g. Nokia CallConnect and iPhone Mobile 8.0) and the CUMC clients as separate solutions that suit different user needs, one being ideal for users who are hardly ever in the office, the other for users who "roam" between offices or are mobile withing their office location (mobile phone used as Wi-Fi client).
It seems however that both can co-exist on the same device. Suppose you run both CUCM and Mobile 8.0 on a iPhone. Would the client give priority to WLAN instead dial by office when the client makes a call while in reach of the WLAN? Is this configurable?
•For Cisco Unified Communications Manager Releases 7.1.3 and 8.0: For deployments that are co-resident with the Cisco Mobile 7.1 client application that runs in conjunction with a Cisco Unified Mobility Advantage server, that client must run without the Dial Via Office feature.
The only thing CUMC provides more is native MP integration with WebEx. However you can use the separate WebEx app for that. See here for automatic VPN access.