The idea of an ishta is far more a primary idea in Smartism than in other sects. It is there in Saivism, Vaishnavism, too, but it's REALLY there in Smartism, so much that some conclude everyone is actually a Smarta with an ishta. This is a false assumption.
Really? Because at my temple, the majority of people are Shaivites and Vaisnavites with Ishta-Devas of their own and everyone (including the priest, who is a Shaivite) says that there is always one deity that attracts practitioners; regardless of their sect.
I get the impression that an Ishta-Deva is not primarily a Smarta idea.