You can try working out example encounters. Assuming 8 situations a day, 5 of which are combat.
Fight vs a SLA using demon and its minions, e.g. Succubus and 2 pet Vrocks.
Fight vs a large meaty hambeast like a giant or an elephant.
Fight vs a dragon using battlefield control spells.
Fight vs a horde of weak creatures.
Fight vs an ambush by bandits with a mage/cleric laying down the battlefield control.
Intrigue at court, where they need to find the stolen Treasury key.
Dungeon crossing, where, pursued by a horde of kobolds minutes behind, they must cross a large moat filled with piranha.
Trapped hallway, where they have under an hour to cross while the kobolds behind get rafts and bridges. Hallway begins and ends with a locked door.
Ask how each character could help the party in the situation, emphasize helping the party resolve it instead of solo-solve.
In the first fight, a trio of flying opponents, one which uses a charm SLA, is going to be difficult for any melee based character to combat. Worse, they're probably going to get charmed. The spellcasters can boost the melee guy to fight in the air, or directly assault them magically, or summon X to deal with it for them. Damage reduction makes it worse if they aren't prepared.
In the second fight, a large meaty creature, would seem ideal for a fighty type to take on, so they charge it....and if it survives to the next round, with a full attack most of their hp is gone. Spells instead stall the creature so it can be plinked to death with impunity. Rinse, repeat.
The third fight is just to illustrate how screwed the party can be. With the casters probably already out of half or more of their highest level spells, the dragon delivers a fear aura, then drops a large Fog spell to cut off vision(but ahh, the dragon has blindsense!) and then blasts with breath and closes to claw. Even the casters are going to be busy just negating what the dragon is doing. What do you do with a creature superior to you in melee, hiding behind invisibility or a smokescreen?
The horde? The fighter is going to cheese them so hard they will reek of cheddar in their next life....or you could drop a Wall of Fire/Black Tentacles or a pair of Fireballs to clear them out. But its best to let the fighter have his fun for a while.
By the 5th fight, the fighter's been out of hp for 2 fights prior sans healing, the skillmonkey is likely wounded as well, the casters are....down maybe their highest level of spells and half the level below. They still have enough slots to push their way through the 5th encounter, or heal the fighter and skillmonkey so they are fresh enough to do the same.
And then you get to the meat of the matter, intrigue. You can have the social master rogue sweet talking everyone. The mage waits for the rogue to identify who likely has the info but won't talk, and then uses Divinations, Charms, magical spies or Speak with Dead to get it out of them. The fighter and the monk sits it out. They have neither the skills nor the stats to do more than make things worse.
The moat now, the rogue might climb the walls along the side and tow a rope. The casters might conjure a temporary bridge, improvise a Cold spell to freeze the water and run across, deploy levitate, or teleport even. Or they could dump a Wall of Stone behind them and buy some time while their pursuers go get some picks. The fighter types...they sit it out, or go make a Last Stand
The traps now, is where the rogue shines, while the casters can detect or remove them, it takes resources per shot, so assuming the rogue knows what hes doing, the party can get past at no cost but some time. The door can be forced or bypassed or whatever.
The combination as a whole should be demonstrative. The casters never need to sit out a situation, and for wizards and clerics, if they find themselves inadequate, they can rearrange their loadout. Rogues generally can contribute, but at the cost of some investment.
And anyone using the magic item argument is answered with, "but the guy who can do it by himself can do it better with the item, or spend the money on some other item"