I was researching the boards on metamagic rods, and found this thread. The following is from the PRD, and the paragraph on the sorcerers and bards explains the rationale quite well:
Wizards and Divine Spellcasters: Wizards and divine spellcasters must prepare their spells in advance. During preparation, the character chooses which spells to prepare with metamagic feats (and thus which ones take up higher-level spell slots than normal).
Sorcerers and Bards: Sorcerers and bards choose spells as they cast them. They can choose when they cast their spells whether to apply their metamagic feats to improve them. As with other spellcasters, the improved spell uses up a higher-level spell slot. Because the sorcerer or bard has not prepared the spell in a metamagic form in advance, he must apply the metamagic feat on the spot. Therefore, such a character must also take more time to cast a metamagic spell (one enhanced by a metamagic feat) than he does to cast a regular spell. If the spell's normal casting time is a standard action, casting a metamagic version is a full-round action for a sorcerer or bard. (This isn't the same as a 1-round casting time.) The only exception is for spells modified by the Quicken Spell metamagic feat, which can be cast as normal using the feat.
For a spell with a longer casting time, it takes an extra full-round action to cast the spell.
Spontaneous Casting and Metamagic Feats: A cleric spontaneously casting a cure or inflict spell, or a druid spontaneously casting a summon nature's ally spell, can cast a metamagic version of it instead. Extra time is also required in this case. Casting a standard action metamagic spell spontaneously is a full-round action, and a spell with a longer casting time takes an extra full-round action to cast. The only exception is for spells modified by the Quicken Spell feat, which can be cast as a swift action.
