But you would still have to have the recipient of the message as a WLM or FB contact. WLM works fine for contacts you already have in WLM but I found there's no easy way to add someone new as WLM contact from the phone.
I have contacts that use WLM on both their phone and PC but they are not on my WLM contact list. In the People hub, WLM is not an option when I view the messaging options in their tile. One of us has to go into the WLM client on the PC, initiate a friend or contact add request, the other then has to approve it. Only after all this will there be an option on the phone to message someone by WLM.
iMessage is much more simple. You only need to know the cell number or email assoiciated with the iMessage account. You can message anyone on iMessage as long as you knew their number or email.
I also find sending pictures over WLM through the phone broken. It either takes way too long or fails completely and I usually have to resort to switching from WLM mode to SMS/MMS mode to send the pics as that usually goes right through.
Hopefully full Skype integration will fix all this.