As a result of the Supreme Court of Canada decision in Ediger v. Johnston, 2013 SCC 18 (released today), the unique to British Columbia Moore v. Castlegar gloss on Snell v. Farrell is as dead as the parrot in the classic Monty Python sketch.
The BCCA had affirmed that rule in its decision: Ediger v. Johnston, 2011 BCCA 253 at para. 84
 This court confirmed in Moore v. Castlegar& District Hospital (1998), 49 B.C.L.R. (3d) 100, [1998 CanLII 4906] (C.A.) that “where both parties have led expert evidence on the issues of causation, it is not open to this court to apply the ‘common sense’ reasoning urged in Snell”(para. 11).
The SCC’s response at para. 36
 The Court of Appeal’s reasons also suggest that it understood the trial judge to have improperly relied on Snell v. Farrell,  2 S.C.R 311, in order to draw an “inference of causation” (paras. 83-85). Snell stands for the proposition that the plaintiff in medical malpractice cases — as in any other case — assumes the burden of proving causation on a balance of the probabilities (ibid., at pp. 329-30). Sopinka J. observed that this standard of proof does not require scientific certainty (ibid., at p. 328); Clements, at para. 9. The trier of fact may, upon weighing the evidence, draw an inference against a defendant who does not introduce sufficient evidence contrary to that which supports the plaintiff’s theory of causation. In determining whether the defendant has introduced sufficient evidence, the trier of fact should take into account the relative position of each party to adduce evidence (Snell, at p. 330).
I wrote about this issue in more detail earlier, here.
For those who want to know what happened in the appeal: the SCC allowed the plaintiff’s appeal, restored the trial judge’s decision that the defendant was liable, and returned the case to the BCCA to deal with the parties’ damages appeals which the BCCA hadn’t considered since it dismissed the action.
On the merits issues, I think the SCC’s decision is the correct. All one needs to do is read the head-note to understand why.
Added on April 6, 2013
While the central issue in the case was but-for factual causation, there’s nothing in the reasons that should be taken to add to, subtract from, or in any other sense modify anything the Court said in Clements.