So many plumbers out there are always anxious to create a new house call regardless of the actual need for it.
I do believe that he or she just wanted to have a come back call and just replace the valve and drain the water and charge you $150. Its like some auto mechanics that say you need new spark plugs but never replace your old ones.
Why, as Joe P stated, would they initially place a faulty valve in a hot water heater if it was not safe?
No offense, Freed or five finger (whatever), ask some questions of others before you take advice for granted from some plumbers out there.