As for "DM won't allow that" or whatever, hey .. there's a homebrew forum around the corner.
I'm here to discuss the rules.
The whole point of exercises like this is to find broken stuff.
By definition, if something is broken, it needs fixing.
One of the functions of rules forums are for discussing and refining understanding of cases so we can get WoTC to issue effective and sensible errata kthx.
I completely agree with everything you've said. But where the rules fail us (and this is but one example), the DM has to make a judgment call.
If we're lucky, in the future this will be fixed. Sometimes, though, we don't get errata for problem rules. If that is the case, then the intent of the rule is just as important, if not moreso in these situations.
As I said earlier, I think this works
if a warlock power has the *Weapon keyword. Why? Two main reasons, I think. While a dagger is a necessary component for gaining the Daggermaster's critical effect, it is not a sufficient one. By that I mean, you've got to also be
attacking with it; in other words, using a power or attack that has the Weapon keyword. I also think that while a pact blade can be both a dagger and an implement, it does not necessarily act in both capacities simultaneously. Most of the time it's one or the other. Either you use it to stab someone, or you channel a spell through it. Most of the time, you are using it as either a Weapon or an Implement. The crit chance increase is a feature of using the dagger as a Weapon, and therefore should not apply when it is used as an Implement.
I think that's a pretty logical explanation of my ruling, rather than a simple "I don't like it cuz it's cheesey!" or whatever variation you prefer. The fact of the matter is, players find new an interesting ways to work the rules all the time. I as a DM need to be ready to respond to those interpretations, and can't always wait for Wizards to make that decision for me.