...
Is the feat useless to anyone other then a sorcerer using arcane spellsurge and/or an invisible assassin? Yeah, it's useless.
The fact that your interpretation of the feat makes it useless is a powerful argument against it. It essentially deletes Invisible Spell from the game, which is generally speaking, bad interpretive practice.
City Magic, Coersive Spell, Cooperative Spell, Deafening, Delay Spell, Echoing, Energy Admixture, Enlarge, WIDEN, Fell Frighten, Metanode, Reaping, Sudden Quicken, Transdimensional, Umbral, Wounding, Violate.
All are metamagic feats. All are feats of very limited use and practicality. Stating something is next to useless is not something for or against an argument. True, you can give me specific examples of the above feats being useful in specific situations, but if your PC had only two feat slots for metamagic feats, you HAD to take metamagic feats, but you could take ANY metamagic feat, do you really expect me to believe you would willingly take any of these?
Unless you are an assassin who loves being invisible, or a sorcerer planing on Arcane Spellsure, Invisibility is of little practical use.
EDIT: also, your reading of "all other aspects of the spell ..." isn't right, I think. You're reading that nothing else in the spell changes. To wit, in the case of Summon Monster that the target line does not change at all. But, it doesn't change except for the fact that it has no visual manifestation. You've chosen to read out the "all other aspects" phrase of the effects line.
Furthermore, the spell description goes on to say that the "effects" of the spell are unseen. What would the "effects" of a Summon Monster spell other than the summoned creature?
I see another feat that specifically is created for the purpose of doing exactly what you want to do. Imbue Summoning. Why would I EVER take THAT feat which requires me to cast TWO spells and add +1 level to one of them, to do the same thing Invisible Spell could do for +0? Imbue summoning sucks as it is. You would Gut Imbue Summoning to make Invisible Spell Stronger.
Summoning: A summoning spell instantly
brings a creature or object to a place you designate. When the spell ends or is dispelled, a summoned creature is instantly sent back to where it came from, but a summoned object isn’t sent back unless the spell’s description specifically indicates this. A summoned creature also goes away if it’s killed or if its hit points drop to 0 or lower. It isn’t really dead. It takes 24 hours for the creature to re-form, during which time it can’t be summoned again.
When the spell that summoned a creature ends and the creature disappears, all the spells it has cast end. A summoned creature can’t use any innate summoning abilities it might have. It refuses to use any abilities that cost it experience points or would cost experience points if they were spells. Like a calling, a summoning is a two-way trip for a creature.
Now, if it said, "CREATED a creature or object", I'd agree with you. But Summon Monster is merely bringing the critter from somewhere else. Unless the critter is invisible to begin with, the critter won't be invisible HERE. The spell is transport, nothing more.
Think of the spell like a bus. The Celestial Hedgehog gets on the Summon Monster Express and appears in the square you point at. Now, if you use invisible spell, the bus is invisible, but the Celestial Hedgehog is still visible. He might be surprised to be zipping through the Astral Plane like Wonder Woman in her invisible jet, but the BUS is the spell, not the monster.
Invisible Spell is next to useless for most Conjuration Spells. It's much more useful for evocation, illusion, and certain necromancy spells. An Invisible Spectral Touch Spell would be a scary thing when combined with Vampiric Touch.
But Conjuring Invisible Critters? No. Sorry.
However, please, I'm always open to discussion. If You can sum up your position in a paragraph or two, I'll include it as a Minority Opinion in the Metamagic Section of THE SPELLBOOK.
EDIT: It just occured to me a use for an invisible summon monster spell. I remember an adventure where when you summon a monster, an effect was triggered against the caster, in a certain location. I think it was lightning or some damaging effect. An invisible Summon Monster would be "invisible", so the detector would not "see" the summon monster spell, thus allowing a conjurer to summon monsters without triggering said side effects. Wish I could remember the module.
