Title says it all, really. A couple reasons: Some people want to fight each other with their low leveled characters to compare how they measure up against the other person's team. Some people want to pull out their level 20 Ultrawizards and fling so many spells that the ground explodes with power beneath their feet. If this is implemented, there's a very real chance I would try to run an actual "Campaign" of Cardhunter, provided I could find some people willing to go through with it.
That's a very effective way to get my attention I've been thinking about ways to implement this - it's a little tricky to do entirely within the multiplayer context, but... ...how would you feel about a custom scenario tag that makes the game load your campaign party rather than your multiplayer party? This way players could agree beforehand what level they'll be, use a campaign adventure to de-level their party as appropriate, then away they go. We could probably also add tags to set character level in these scenarios, but they may just specify health and not enforce item limits. I'm not sure on that point yet. Would this fit the bill?
Sounds like it would work perfectly for all of the scenarios I suggested! If the theoretical campaign is starting from low levels, it would probably be best to have item limits in place as well so that you don't have to deal with quite so many unreliable blocks and little zaps in your deck at level one.