When an accused chooses to testify, the accused takes on the obligations of a witness in the proceeding and must tell the truth, the whole truth and nothing but the truth about the allegation.
The accused does not enter the witness box as an advocate. The accused is not there to answer tactical questions about the conduct of the defence or to argue the defence theory of the case.
For example, it is impermissible for the Crown to ask the accused whether he intends to call other witnesses or to explain why other witnesses are not being called by the Defence (see R. v. Bouhsass,  O.J. No. 4177 (C.A.) at para. 12 where the Crown improperly required the accused “to explain why certain witnesses were not being called to testify and …to answer for the fact that his evidence was not corroborated by anyone...”).
Such questions are not only irrelevant but they may also infringe upon solicitor-client privilege by compelling the accused to divulge tactical discussions with his counsel.
Such questions also encourage a reversal of the burden of proof by insinuating to a jury that the accused has an onus to corroborate his evidence.
The same point was also made more recently in R. v. Dhaliwal,  O.J. No. 4551 (C.A.) where the trial judge improperly allowed the Crown to cross-examine the accused on his “theory” of the case over the objection of defence counsel. The Court of Appeal comments at para. 13 that:
...asking the appellant, in front of the jury to provide his "theory" of the case or to explain the evidence against him undermined the presumption of innocence. Permitting the Crown to ask the question, and requiring the appellant to answer it, could only have led the jury to believe that he had some obligation to provide a "theory". The line of questioning should not have been permitted.