And they have fewer spell points. And your notion that they can hit the same DC is fallous:
Overview
Bard DC: Charisma+ spell level + enhancement + item + feat
Wizard DC: Intelligence + spell level + enhancement + item + feat
Feats
Bard feat slots at 14: 4 (5 if you are human)
Wizard feat slots at 14: 6 (4 character slots 2 metamagic slots, *1 more if you are human)
*Corrected. Thanks MT.
Maximum number of schools to apply focus to (spell focus +1 and greater spell focus +1 (for a +2 total)
2 spell schools
2 spell schools. Leaving 2 additional feats for metamagic uses such as extend, height, etc.*
*Corrected. Thanks MT.
Stat Bonus
Primary stats: Typically this is lower than a Wizard's Intelligence because most wizards don't need to spread their points to as many things. More bards require dexterity, con, and strength than a wizard. So, on average:
Bard average Charisma: 26 with full bonuses.
Wizard average Intelligence: 30 with full bonuses.
Spell Level
As has been stated before, most Bard spells are lower level for the same spell than Wizards or Sorcerers. This has the benefit of a reduced cost (by a very small amount), but a lower save DC as well:
Mass Sugestion:
Bard: 5th level. base save DC 15. Cost: (I believe) 30sp
Wizard: 6th level. base save DC 16. Cost: 35
Item Bonus
Irrelevant as both classes can use the same items to the same effect.
Enhancements
Also Irrelevant. The only enhancements that these classes have to boost DCs only do so on wands. And for those, they both have access to them!
So at the end of the day, Bard DCs can be anywhere from 0 to 6 points lower. This assumes that the Bard runs away from the party every time they buff their spell save DCs. I don't think giving the Bard +1 DC to his spells, +1 to others, and more spell points, is going to break the game, nor shun others in favor of a bard.
And, at the end of the day, if people go for MORE diversity in a group (including a bard) as opposed to less (more Wizards, Sorcerers, Fighters, Barbarians, Clerics), isn't that a good thing?