I have been in an unsuccessful dialogue with the QCA to determine the reason for this somewhat bizarre decision in the light of internal assessment being removed from most other subjects, for the very reasons that your correspondents cite.
The QCA informs me that its decision was based on "consultation with the subject community". This, it appears, was through a survey conducted via the QCA website, the results of which are now posted on the site. The data is riddled with errors and is most certainly not statistically viable. Such conclusions as it is possible to draw from these data suggest, in fact, that a majority of respondents were not in favour of compulsory internal assessment.
While I agree that for some students in some schools coursework might be the most appropriate form of assessment for A-level practical physics, there are a large number of schools and colleges for which it is not. We need to keep the options open so that schools can choose the best form of assessment for their students. I therefore urge A-level science teachers to register their disapproval with the QCA and ask it to reconsider its decision.
Graham George. Chief examiner (A-level physics) Edexcel London WC1