3 S.C.R. 6,  S.C.J. No. 85, at paras. 21-24, the Supreme Court of Canada set out the underlying rationale for the limitations statutes as follows:
There are three [purposes], and they may be described as the certainty, evidentiary, and diligence rationales. . . .
Statutes of limitations have long been said to be statutes of repose . . . The reasoning is straightforward enough. There comes a time, it is said, when a potential defendant should be secure in his reasonable expectation that he will not be held to account for ancient obligations. . . .
The second rationale is evidentiary and concerns the desire to foreclose claims based on stale evidence. Once the limitation period has lapsed, the potential defendant should no longer be concerned about the preservation of evidence relevant to the claim. . . .
There are, however, rules for the amendment of pleadings, which allow a court to relieve against the harshness of an expired limitation period in certain circumstances (for instance, in Ontario Rules 26.01 and 5.04(2) of the Rules of Civil Procedure). It would be wrong, however, to view such rules as allowing a party to use the existence of an outstanding claim, and nothing more, to defeat the protection of relevant limitation periods.
See Frohlick v. Pinkerton Canada Limited, 2008 ONCA 3 (CanLII) at para 20
In an upcoming blog I will discuss how this judicial policy informs the rules in respect of the amendment of pleadings.