I'm actually with the OP on this one. I don't like this tactic and I normally LOVE Swordmages for the whole kiss-and-tell schtick. Consider this a second for interwyrm's idea - if the Swordmage hits and then decides to leave the combat, I would rule 'end of the encounter' for the power. What does leaving the combat mean? Clearly that's pretty subjective, but I think it would occur at the end of the swordmage's turn when all of the following occur:
1) He (or she) leaves the area that the marked creature could affect in a single turn (including moves and charges)
2) He is not engaging any enemies
3) He is not within (Speed x 2) of any allies
4) He is not attempting to provide direct support to an ally outside of Reaper's Challenge.
Or in other words, if the swordmage moves somewhere where he could take a short rest, the power should end.