Why would you not use SharePoint? Why would anyone not use SharePoint? You turn on the server, and there it is. Except for RWW and Exchange, SharePoint is the biggest reason we stay with SBS.
I don't personally know what SharePoint does that would help our office. This is largely because I don't know much about SharePoint, and partly because I'm just not aware of any "itch" that needs scratching in our flow.
Maybe because I'm IT support and not on the ground, and the MD and staff aren't very computer savy so they don't articulate any particular need. But basically we set things up as smart as we could at the time 4 years ago, and since then if I get complaints it's because something is broken or something is slow, that's about it.
Probably we are in a local minimum, where stuff seems good but over the next hill there's something way better. I will have to keep an eye out for new stuff more, I've been letting it just ride a bit.