I used to work for a large software corporation who deployed Lotus Notes (after being acquired by an even-larger corporation who mandated its use).
I really didn’t like Lotus Notes, and many other employees really didn’t like it either. Emotions flared, ranging from outrage to hatred. Our consensus was that Lotus Notes is a big step back from the open-standards goodness of email. (I feel soooo sorry for Redhat right now, if they are being forced to use it internally by their new owners!!)
I think the long and short of it is that Open-source-friendly people are quite likely to shun closed-source, proprietary software, such as Lotus Notes.
But having said that, no doubt money is to be made by developing such integration, so who am I to stop those developers from making a living, by advocating such integration?
I’d personally never even dream of using it, not wanting to touch it with a ten-foot pole. But large, rich, stogy corporations and institutions who are deadlocked into existing, legacy Lotus Notes installs, might indeed have some use for it.