From what I remember people saying at shadowpriests.com (you could look it up if you wanted), most DoT timers take the time you started casting, "guess" when it will land based on your latency, and then display a countdown irrelevant to whether or not the debuff is actually there. If you have multiple shadow priests, they will track OTHER shadow priests' debuffs as your own, and because of the "guesswork" with when it lands, using /stopcasting macros throws it off every-so-slightly. Something along those lines.
ClassTimer and Dotdotdot use a different method (API - no idea what it means :P ) to actually track YOUR debuffs, exactly when they land, when they get pushed off, and so on. There isn't any guesswork, and isn't any confusion as to whose debuffs are whose (except Shadow Weaving it seems - it doesn't track SW if another spriest lands it first - though it's hardly a huge issue).
EDIT: Plus you can force ClassTimers to show other debuffs or buffs on yourself - I've got it showing CoS and ISB on the target, and any trinkets/Bloodlust/MSFD proc that are on me. Since my eyes are always right on the timers anywho, it's nice to be able to watch them and not have to glance over at my buff list. Makes short-duration procs much easier to see as well.
Edited, Aug 21st 2007 12:03:22pm by lsfreak