I think that swearing is something that a therapist should ideally alter to be in keeping with the clients terminology in the same way that a therapist should ideally alter their terminology in other areas.

Probably better for a therapist to not swear and not offend than to swear and then offend. I swear a little. My therapist doesn't swear at all. So I try and swear less. I've had therapists who have seemed happy to reciprocate my frequency, though.

I guess it is most effective when used in moderation. Individual decision, I think. There are (or there certainly can be) other forms of expression that convey a similar emotional tone - but they can be hard to find at times.