Yup, they are NEVER useful. Not that spy-archetype rogue that my friend plays in PFS that regularly gets self-perpetuating bluff up to 40 at level 4. Nope, he hasn't saved the party's butt ever. Or that pirate/scout in my skull and shackles game who can charge in any line she wants for sneak attack damage. Never useful ever, nope.
SPAM is useful too. It has decent protein content. It's just not pleasant.
Okay, good. Good to know that my utility wizard is pointless and hasn't saved the party's butt on numerous occasions because he's smart and preps the spells he needs ahead of time, like hold undead, or heroism, or whathaveyou. Weird, I thought that existed, and that I DIDN'T blow through all my spells every day so that the extra-spell a day thing would have been nice, but not required. Good thing you set me strait (eyeroll)
Sure, he's a wizard. He'd be even better as a specialist, though. As long as you're only preparing one opposed school spell per level you'd have at least as many spells and usually more. Taking a big focus school like conjuration and opposing small schools like necromancy and redundancy filled schools like enchantment is cheap and you can still prepare hold undead and heroism and come ahead at every other spell level.
-Swashbuckler with 7 Str, fencing grace, weapon spc, level to damage, etc.-Tengu Slayer with TWF and TW Feint, sneak attack, etc.
-Shield-bashing tank slayer with improved shield bash, shield slam, etc. I don't do a lot of damage, but I make sure people don't get near the squishies by bashing them away.
Your swashbuckler was almost certainly illegal at low levels. Adventuring kit is heavy. Even 11 strength is low for a light armor character and that's just two buy points from qualifying for power attack.
Your tengu slayer should have power attack. Lots of things are immune to sneak attack. Lots of things that aren't immune to sneak attack will have feint DCs that you won't always hit. Nothing is immune to power attack. If he also cheated on the encumbrance rules and lacked strength he should have had piranha strike, which is power attack by another name.
Your tank slayer should have power attack. He really should have power attack. Really. Even if you never intend to use power attack this character should have it. Power attack is a prerequisite for improved bull rush. You're sabotaging the reliability of your main tactic by not taking the improved maneuver feats.