Was the booster of the interplanetary transport system ever considered as a super heavy launch vehicle for commercial payloads onto LEO and GEO? (with an expendable or reusable second stage) Or its sole purpose is to enable Mars and outer SolSys exploration?
It is unclear at this point since everything has been only notional so far.
Musk has said that they will try a 9 meter diameter, 21 engine interim ITS first now, (original was 12 meters, 42 engines). This is clearly aimed at maturing the technology and being cheaper to develop and test.
Additionally, SpaceX has a 4425 satellite constellation (CommX) they want to launch. Falcon, (9 or Heavy) are volume limited in their fairings, and even though the Falcon Heavy could launch a lot of mass, the number of satellites is still going to be volume limited. A hammerhead fairing can only be so wide, before it stops working aerodynamically, and at 5m wide the fairing is already sticking out a meter on either side of the Falcon core booster.
The speculation (Hoping to be confirmed Sept 28th in Australia when Musk is scheduled to speak at the IAC) is that they would test the baby/mini ITS as a satellite launcher and fund it, through launching their CommX constellation.
The CommX FCC application PDF is here.