I imagine that being trained by a ppl licence holder would scare a lot of new students, not something I would fancy. The CPL licence holder has proved him/herself able to fly at a competent level for public transport with a CAA examiner. I also agree that an Instrument Rating should be mandatory for every flying instructor. The student should feel confident in his/hers instructors ability. I never used to enjoy flying with a fair weather only flying instructor, until I got my own IR.