I think it's sort of the opposite (of acquiring ET for their customers).
As a company "born Cloud", Salesforce have had to make sure their integration capabilities have been up to snuff, and other vendors have been able to leverage those integrations to reach Salesforce customers via the AppExchange (similar to the App Store).
So the capability of using Salesforce whilst using other apps with integration has been a capability for a while, and so customers could tailor their own solutions to their needs, budgets etc. Except that more and more, customers want the one-back-to-pat/one-neck-to-strangle, and the associated 'one-solution, no integration' benefits. Hence Salesforce have been parking their tanks on the lawns of service providers. When asked for features, Salesforce response is sometimes "There's an app for that", however sometimes the demand for them to provide certain functionality is so great, or the opportunity large enough, that they'll enter a market in this fashion. They're currently doing something similar with identity management. Ping Identity had a solution they could sell you, but soon they'll have to compete with Salesforce Identity.
Competing with someone who hosts the platform you're working on is possible, but it's definitely a disadvantage.
The other factor here is what the other players are up to - Oracle recently purchased Eloqua, and for those of us in the Salesforce line of work, an acquisition like this has been expected for the last few months. Email marketing is an area in which Salesforce has been weak for some time, they didn't want to do it because it's not their core focus, and hey, "there are apps for that". But once Oracle moved, Salesforce were going to follow suit to block that advantage.