@z428 Now we're talking about the app level and not the protocol level. The GMail webapp still uses multiple protocols; it's more akin to having a GTalk pane next to your GMail pane. Likewise, you can build an email client that had XMPP chat embedded in the side (e.g. Thunderbird, Evolution, eM Client, and so on).
I wouldn't call either part of that an "open silo". Even then, the only "silo" aspect is in incompatible data representations -- you can still build a converter or bridge as you said.