Gayford and all. I would be curious to know how common it would be for ATC to only use secondary RADAR.
Not uncommon at all in areas with civil ATC where it's very unlikely that unknown aircraft would appear. That doesn't stop the mil having primary radar coverage too if there is some threat perceived but civil ATC often does not have easy access to the primary picture. The benefits are a 'cleaner' picture and cost savings by not having to install and operate a primary radar.