If you are happy with the grades I'd leave well alone and not change anything! I honestly think it depends on who you get - same as for every other exam and with no disrepect to those examiners who are experienced and know exactly what they are doing I am convinced that there are a few rogue ones who don't or who with an oral exam could put pupils off. Doing the oral ourselves although we don't mark it as such we can mark the recording unofficially so we know what we think the candidates should get which gives us more ammunition if there is a dispute about results. Also our pupils much prefer to have someone they know who knows how to get the best out of them as indiviuduals. So I'm prepared to put in the hard work that is required as I think they feel more at ease with it. The upside is that it is much easier for them to be judged by necessity as more "spontaneous" with an external examiner.