To me, it seems kind of arbitrary, since it doesn't take your saving throw into account. For instance, the flametouched mace sets you on fire and causes you to burn every round, without ever having to 'get past' your saving throw score. But I guess you did have to fail an 'AC save' in the first place in order to get hit. Yeah, ok, I guess I'm fine with it. As an alternative to fixed-length durations, it seems more interesting.
On a side note, does anyone else seem to think that 4E is going to fit very well with using index cards to keep track of things? Like, an index card saying "Flametouched. 5 fire damage per round. Save ends." or "Pit Fiend Poison. 15 poison damage per round. Weakened {here you would include a short description of Weakened}. Save ends." Just hand them to the affected PCs. Since there are no durations to mark down, they would be reusable. I dunno, maybe other people have been using index cards forever - just seems like somehow things in 4E are particularly suited to it. Maybe it's because there are lots more 'on/offs' than 'countdowns'.