The difference between the APIs is that anything you do with the Skype API is limited to the Skype client.
With Google's API, you have unlimited options. The only limit, really, is your imagination.
You can integrate GoogleTalk VOIP network with any third party application. That's why people are saying that Google has the potential to outvoip Skype.
Click here to SPEAK to an agent
For instance, if you wanted to build an app that would enable someone to chat with one of your sales agents on your ecommerce site, then it may be possible to build that application and utilize the GoogleTalk network with whatever client you wish to use.
Add this to Local Search
Or perhaps you can build a local search network and integrate the power to instantly speak to those listed in your directory. Imagine being able to deliver these kinds of leads: Someone is browsing for restaurants and can click a button and instantly place an order or make reservations, make hairstyle appointments, or even order children's shoes from a local merchant and pick them up on your lunch break.
Teens can see where they're at AND call
Other people have suggested that you can mash up community maps with voip so that you can click and speak with friends on the map. Sounds like a thing teenagers might be into. Can it be dropped into a mobile phone? Yikes!