This discussion arose during the last session due to Iawa(sp?) being able to Death Mark something while invisible and deal 2d6+6d6+6 and 6d6 spread damage per encounter in one blow. Here are some reasons why this is completely within class balance.
#1 Armor Class
For any Strike manuever to work, it has to hit the target. AC is the easiest statistic besides skill bonus to increase magically at (1000x)^2gp where x is the amount of the armor bonus. Add in that Swordsage has a medium Base Attack Bonus (ala Cleric or Rogue) and you'll see why it's the "skirmisher" of the Tome of Battle classes.
#2 Recovery of Maneuvers
Swordsage has the worst recovery mechanic, only being able to recover one manuever for a full-round action in combat. Swordsage also has the least number of maneuvers readied at any time, but instead have a high amount of maneuvers known.
#3 Combat Resilience
Swordsage also has lowest hit die of any Tomb of Battle class, they're only saving grace is having a Monk Armor Class. (10+Armor+Dex+Wis+Misc) This again enforces their role as "skirmisher".
#4 "They Deal More Damage Than Wizards"
Wizards were never meant to deal damage. (Sans Evokers, which suck at resource management.) They control the battlefield so that their allies can come in with advantages and their foes with disadvantages. Fireball was called in as the "Arcane Death Mark". Let's take a look at the two.
Fireball
Evocation [Fire]
Sor/Wiz 3; requires V,S,M
Cast Time: Standard action
Range: Long (400+40'/Lv)
Area: 20'-radius spread
Saving Throw: Reflex (Half)
Effect: 1d6/Lv (max 10d6) fire damage in the area.
Death Mark
Desert Wind (Strike) [Fire]
Swordsage 3
Initiation Time: Stanard action
Range: Melee
Area: Double area of target's square
Saving Throw: Reflex (Half)
Effect: 6d6 fire damage to enemy and all within the area, with the exception of you. Supernatural effect.
Advantages for Fireball: Range (600' at the least), lack of attack roll, higher damage cap, metamagic able.
Advantages for Death Mark: Higher initial damage, can't blow yourself up.
Disadvantages for Both: Anti-magic aura kills it, fire resistance/immunity kills it.
FYI Death Mark + Emerald Razor does not work since both are Strikes and they cannot be overlapped except when indicated as a class ability. Cloak of Deception does work however, since it is a Boost.