I'm a big fan of using S2 wherever I can, but I'm a fairly recent convert to that way of thinking...
For me, the better error-checking that comes with S2 is a bigger win than any potential impact due to increased messaging (and I'm not even sure that's really a thing - I've certainly never noticed any performance impact if it's true).
I have some far-flung sensors that occasionally drop down to a 40kbps connection - since they're paired S2, I sleep better knowing that S2's better error-checking ensures those devices are still able to talk & listen fine.
The one thing that always held me back from using S2 was that HE's built-in firmware updater doesn't work with S2 devices, and I didn't want to mess with z-sticks to deal with that.
But since I came up with these update steps, it's been smooth sailing - I've done lots of firmware updates using this process (on both S2 and no-security devices), and they've all gone very smoothly.
