Voodoo already hit it. Sublime Chord was the bard caster PrC and it was much more powerful than the DDO caster PRE with spellsinger.
The Sublime Chord allowed bards to add spells from the sorc / wiz list and up to 9th level. Song of arcane power provided a caster level bonus up to +4 based on perform check. Metamagic song (races of stone) allowed for the swap of bard song daily uses for free meta enhancements per use. There were several songs which provided bonus to DC`s in various supplements.
This also made it much easier for a bard to qualify for epic spells.
Adding enhancements to allow bards to add some sorc / wizzie spells in to spellsingers as enhancements has been suggested and follows similarities to sublime chord. I would not be surprised if we do see something along those lines the third tier for spellsinger when it comes out, but not to the abuse possible in PnP. Supplements could ruin a campaign fast with casters. Limiting the spells to enchantments and illusions would follow the theme better without adding high nuking and mass instant death.