I use XMPP/Jabber a lot without using Shmoose or any other fully-fledged XMPP client apps. However PingYou has proven useful, because you cannot add XMPP contacts using out-of-the-box SailfishOS 
. Once you have done this, those contacts appear in your SailfishOS contacts, where you can connect them to existing contacts if needed.
When this is done, you can use XMPP/Jabber to send text messages just the way you do it via SMS (which way you use to send the message is metioned below the message field and you can also switch between the available methods there), which I find pretty cool.
The downsides of using XMPP/Jabber like this (mostly referred to here already), are mentioned and discussed here.
I really like the idea behind the XMPP integration in SailfishOS, but - like many others around here - I think it’s high time Jolla finally went the last few steps to really make it useful for everyone!