The marshal class is prone to far more abuse than the bard is, simply by virtue of the ease with which a skillful player can abuse the marshal abilities. Marshal is the only class in my home campaigns that I ban out of the whole 3e lot (including ToM and MoI). It is that bad. Imagine PCs with +10 to initiative and +10 to damage when flanking. It's silly. That is not to say that the bard cannot be abused either (it can: fascinate is one of the most powerful abilities in the entire GAME), but the bard abilities tend to be more situational and not as universally useful as the auras of the marshal, hence, playing the bard tends to be a bit more difficult.
Now the marshal class is a great idea with a poor implementation, one of those looks good, plays bad things. The bonus never should have been based on the marshal's charisma, or at least there should have been some kind of limit placed on its uses. If you would like to use the auras in place of bardic abilities, it's not a bad idea, but you need to be circumspect about other things this will affect, and I recommend house-ruling the way the auras work (i.e. don't base the bonus off Charisma). Find another way to make Charisma important. Here are some suggestions to "fix" the marshal auras. Some of these are mutually exclusive but you can mix and match to taste.
-A marshal's minor auras grant a bonus equal to +1 per three levels, maxing out at +6 at 18th.
-A marshal can provide the bonus from his auras to a number of allies equal to his Charisma modifier (minimum 1).
-The radius of a marshal's auras is equal to his Charisma modifier x 10 feet.
-A marshal has a limited number of uses of his auras. Activating an aura is an immediate action. A marshal can activate each of his minor auras a number of times per day equal to 3 + his Charisma modifier and he can activate each of his major auras a number of times per day equal to his Charisma modifier.
-A marshal can use each of his auras a number of rounds per day equal to his Charisma modifier. For example, a marshal with a Charisma modifier of +3 who knows 2 minor auras and 1 major aura can use each of those auras for 3 rounds per day, divided up however he wishes.
Also, be aware that bardic music is not the bard's only ability. Bard's also have a better skill list than the marshal and spells, which means that the marshal auras in relative power terms are better than bardic music. You might want to limit bards to minor auras only.