I've not gotten a whole lot of requests to integrate Google Checkout into CFWebstore, but I do get them once in awhile. Why do merchants want to use Google Checkout? A couple reasons. First, like PayPal, they don't have to pay for a real merchant account. Particularly for smaller merchants, a merchant account can be a considerable expense and also requires having a business checking account, most of which have a sizable minimum balance on them. Google also provides incentives for using their service, such as giving a certain amount of service free when you use Adwords. This can make it even more financially attractive to merchants who are looking to cut corners.
So why do I not include it? There are a couple reasons I generally steer people away from Google Checkout. First, the API is very difficult to work with. It is very complicated and does not work like normal payment gateways. In some cases, it won't accept values passed from the cart, so you have to do additional configuration on the Google site and try to make sure it all matches with your store. Unlike PayPal, Google does not give you the option to just send a total amount for the order, you have to provide all the detailed information on the order. So in situations where the API doesn't fully handle a feature, you run into issues with being able to support it.
One of the biggest issues I've seen though is many customer complaints about this service. Just do an internet search for "google checkout complaints" to find many of the issues and problems customer and merchants have had. It's hard for me to really want to put a lot of effort into supporting a service that is not serving their customers well. PayPal does not exactly have a stellar record in this area either, but they are a much more mature service that is practically ubiquitous on the web at this point. Still, I always strongly suggest anyone serious about doing internet sales consider getting a true internet gateway and merchant account such as through Authorize.Net or other gateways supported by the software, particularly for the stronger controls you have over fraudulent transactions and direct interaction with the customer.