XMPP seems really great in a context of needing to inter-operate with other servers. However, for our use-case all communication was in a silo of a single site so this was not a major benefit to Drop-In Chat.
Our team wants broad broad browser support (for legacy corporate deployments) and a well-tested client side module. BrowserChannel, which is used for gmail chat, is very widely deployed and well tested.
Maybe I'm missing something, but even if we went with XMPP, wouldn't Drop-In Chat still need a way to stream messages from the server to the client? If so, we would still need to rely on a transport mechanism like websockets/XHR streaming/forever iframes for the in-browser real time message delivery. BrowserChannel/WebChannel bundles up this abstraction nicely and hides all the ugliness of dealing with browser quirks for streaming message delivery.