Same thing could be said for achievements. They have the same effect. You are correct though. I noticed the same thing on Battlefield 3. Every time they release a new set of assignments with some bullshit objective the game becomes unplayable. Like recently they posted one for getting 25 AT mine kills. So now instead of having balanced teams of snipers, suppport, engineers, and assaults, we have 12 v 12 engineer matches where everybody just drops mines everywhere and people are too scared to drive vehicles to detonate them... Challenges destroy the play-style of an online game.
If they're really going to do something like this, they need to make them less level-specific and easier to attain. Double the number of challenges, but halve the point value and make them something basic like (get 25 kills in a single match.) This has no effect on the game itself, but it's added incentive to play better and get something for your effort.
Fear not, my brothers! The next generation of gaming is upon us! Be glad! A reward for all your toil and all your sacrifices is upon us.
There are those who said this day would never come. What have they to say now?
(Yes, I ripped off a quote from Halo 2 and modified it to my own purposes. Sue me.)