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You could (maybe?) use it as a "I am close" metric in your home-automation setup.

Can't you also track and read messages, mobile data and calls that are send and received from the mobile device?

An IMSI catcher is nothing but a 'fake' cell phone tower for a phone.

An IMSI catcher is not a hacked femtocell. It basically just tracks nearby cell phones. A hacked femtocell, which is what you're thinking of, could do this.

No, because those messages are encrypted.

Likely with a semi-busted protocol, depending on whether we're talking GSM/CDMA/LTE/etc, but a pain to attack, regardless.

They are sometimes encrypted but setting up a cell tower proxy (femtocell) let's you disable encryption to connected devices and then you can read any texts sent/received through your femtocell. Watch the defcon/blackhat presentations showing this. First one I remember watching was maybe 10 years ago.

Location targeted marketing (spam).

Yes, tracking data is valuable to ad brokers.

There's probably a way to exploit this for geofencing purposes.

Im interested in seeing if I can use this to help my home automation system figure out how many people are in my house, or at least if Im home.

Attendance verification/tracking?