An application for production under the Mills regime has potential benefits and drawbacks for an accused that must be weighed on a case-by-case basis.
The Potential Benefits
The Mills regime recognizes that the production of relevant information concealed in a private record may prevent a miscarriage of justice. As stated by the Supreme Court of Canada in R. v. Mills,  S.C.J. No. 68 at para. 89:
…the accused’s right (to the records) must prevail where the lack of disclosure or production of the record would render him unable to make full answer and defence. This is because our justice system has always held that the threat of convicting an innocent individual strikes at the heart of the principles of fundamental justice….
A good starting point to appreciate the potential benefits is “Therapy and Other Records in Sex Cases: Some Illustrations” by Alan D. Gold and Michael Lacy, ADGN/RP-060, April 2, 1998 (QL). The authors provide a review of cases where:
…the records were useful in establishing the tainting of apparent “memory”, improper therapeutic techniques, previous inconsistent statements, exaggeration, fabrication and ulterior motives in cases where objective evidence to disprove the allegations was otherwise not readily available (at para. 3).
In a prosecution involving multiple complainants, private records (consider electronic documents such as e-mails or social media posts) may also be useful for establishing collusion.
The Potential Drawbacks
The potential drawbacks of advancing an application for production of third party records include:
1) The application and the associated development of a sufficient evidentiary foundation are time-consuming and will represent a significant financial burden for an accused with limited resources;
2) New information may be uncovered that strengthens the Crown's case, that leads to a new, unrelated investigation, or that elevates the potential penalties (e.g. the record may establish that the alleged offence took place after a date when the punishment increased);
3) The trial judge may review information that casts the accused in a negative light; and
4) If the trial judge reviews the private record and does not order production to the accused, the trial judge will have had access to information about the case that defence counsel has not even seen.
The bottom line is that the decision to make an application for production is never automatic and must involve a careful consideration of the potential benefits and drawbacks for each accused.