Also, look at twin strike for a rogue. Lets say he has two +3 rapiers, weapon focus, and the backstabber feat. Give him Dex 20 and Cha 18, and compare to sly flourish.
For a 50% hit chance (with combat advantage):
Sly flourish: damage = (0.5 hit)*(4.5 weapon + 3 enhance + 2 WF + 5 dex +4 cha + 13.5 sneak) = 16
Twin strike: damage = (2 attacks)*(0.5 hit)*(4.5 weapon + 3 enhance + 2 WF) + (0.75 hit once)*(13.5 sneak) = 19.6
+3.6 damage is pretty decent... that's almost as much as the extra damage you get from sly flourish itself. And at epic it is significantly better since the base damage, sneak attack, and enhancement bonuses will all be higher.
A few problems:
(a) Only a ranger with the twf option could dual wield rapiers. A Rogue could have the double sword from Adventurer's vault and it would be effectively the same thing
but
(b) Twin strike uses strength for melee attacks and dex for ranged attacks. That means that a rogue would not be as likely to hit with the twin strike, since it is very unlikely they would also have 20 strength. Best case scenario, given the dex and charisma you listed is that he'd have 18 str [We'll go with a drow/halfling, that means 15+2 for Dex and 13+2 for charisma, getting the full bonus each time, that would cost 10 points, allowing for 12 points to be spent on strength giving it a 17 str, or if str did get the bonus from leveling, it could be possible to have 16 starting str (9), 17 dex (7) and 16 cha (5) charisma and str each get 1 boost, while dex gets 2 before all three are boosted at 11]. So, in the "best case scenario", the twin strike is only hitting 45% of the time.
Twin strike: damage = (2 attacks)*(0.45 hit)*(4.5 weapon + 3 enhance + 2 WF) + (0.70 hit once)*(13.5 sneak) = 18
It's still up there, of course, but at that point, it may be a good idea to go with the brutal rogue type, improving the sneak attack damage by 4. That would make the difference even greater.
So a level 11 rogue with 18 STR and CHA, plus 20 DEX would do well to full out multiclass into ranger. However, he would need the superior weapon prof and the weapon focus, which along with the 4 feats required for full multiclass, you've filled your allotment, meaning you've taken the backstabber feat at 11 to get this scenario. You are also missing out on two weapon fighting, two weapon defence, nimble blade and other feats that could replace the 4 multiclass feats that can increase the initial damage/chance to hit [or just get better defence, or better initiative which allow the rogue to deal more damage over time].