Mark Deakin and I had a think about this question, posted on a comment by Michael (thanks for the idea Michael).

So we spun up a video call together - I was at home and Mark in the office and a live sharing One Note 2007 page.  Here's what we came up with but please add your own ideas as comments if you can think of more.

Communicator 2007 with OCS is better than using WLM because:

Its integrated with the AD, Exchange and SharePoint

• Looks up all the numbers for you, mobile, landline, home etc from contacts or the GAL
• Uses your calendar to auto set your presence state
• Takes you to their mysite on right click
• Presence globes throughout MS Office system, including SharePoint, AD, Outlook, Word, other apps etc.

Security

• Anyone can sign up for your Windows Live ID and thus pretend to be you (all it says is 'email address not verified'
• In WLM you can change your name to someone else. All too easy to change your name and impersonate someone else.
• When you leave an organisation, your communicator buddy list stays with the company. With WLM you take that list with you
• Communicator is possible to log conversation centrally, not possible with WLM
• Because it encrypts the traffic so people can't intercept your messages as they travel outside the firewall

Higher fidelity presence

• You can see at a glance if they are OOF before you email them or IM them
• With Communicator you have advanced presence such as 'in call' and 'in a meeting' which are set automatically
• You can tag a contact if you are looking for them
• With Communicator you can also see the presence of those that are NOT on buddy list.  This is a big one - its people you don't know that well that you need the most presence assistance with.  I'd have to put the whole company on my buddy list to use WLM for this.

Phone integration

• Communicator integrates with your phone
• Can divert calls to the device you choose even mid call
• Communicator lets you answer the phone on your pc if you like because the two are integrated
• You can have your phone do simultaneous ring like your desk and mobile
• Via remote call control, you can see your phone calls come in even when you are not near your phone or mobile and can choose to take them via the pc or send to vmail
• Is integrated with your voicemail letting you talk to your voicemail inbox from the client
• With Communicator you can make outgoing and incoming calls from your PC. WLM only does outgoing - and its nowhere near as good quality (IMHO)
• You can click on a phone number in a document and it will phone it, you can cut and paste numbers and edit them before dialling

Enterprise management

• Communicator client is updated once every 3 or so years. WLM has a shorter refresh cycle so requires more effort to maintain in an enterprise
• You can do multi person video conferencing - don't think you can do that with WLM
• Can IM a DL
• Cleaner UI, more corporate