Powercast is useful to anybody with magic, but only "essential" to a few classes, particularly the more offensive spellbooks (Psionics and Wizardry).
Magic falls off on Ascension Peak. Enemy spawns match the party's average level, you can never truly master AP, like you can with the other 95% of the game. Even if you somehow ground your party to level 50, you would face similar enemies, such as level 50 ArmeGazers.
This leads me to my last MDP, consisting of a Bard, Gadgeteer, Priest, Alchemist, Psionic, and Mage. At AP, the Bard and Gadgeteer were stamina-casting at whatever level they felt like, not consistently doing anything to enemies near their level (they did good against lower level enemies, however).
Even the four casters with maxed Powercast weren't doing much against same level enemies. I had to rely on ranged combat.

This gives credence to the main 4 attributes to max: Strength, Dexterity, Speed, Senses. You get Powerstrike, Reflextion, Snakespeed, and Eagle Eye.

The end-game is indeed determined by ranged combat, if you're a grinder like me.

Fro other posts, it sounds like you were playing the game at far higher levels than intended.

Up through level 24, your casters can keep up with enemy levels due to a combination of your level increasing, your safe power levels increasing (and spell level increasing up through level 18), and Power Cast. The problem is that, once you're past that point, you have no more spell or power levels to gain, and hence your spells can fall behind.

At more reasonable levels, the game is more balanced. It perhaps isn't a surprise that game balance falls apart at higher levels.

Also, don't forget that there are spells that benefit from Power Cast but don't target enemy resistances, like Heal All.

(If I were making Wizardry 9, one change I'd make is to make damage spells not be so heavily dependent on character level, and make it so that damage spells are rarely ineffective. This goes both ways; I want enemy damage spells to still be a threat in large numbers, even if the party has Element/Soul Shield up. There'd only be so far that you could mitigate the damage from such spells. As for the CRPG I'm going to make, the mechanics will not be similar to Wizardry 8, and implementing something like Wizardry 8's level difference mechanic would not be feasible simply because the design of my CRPG excludes traditional leveling, so there's no "level" stat that could be used.)

Bishops are one of the characters where you have to spend extensive time grinding their skills. In particular:
Their melee combat is worth it for Staves of Doom (1 guaranteed, others can be farmed from Rapax Corpses).
Their ranged combat is worth it as well for Wrist Rockets (guaranteed from Crock before you kill him), and good stones can be readily bought.
Their magic is tedious, to say the least, but at least they have the versatility of four spellbooks and 6 realms, ensuring that they always have something useful to cast.

The hybrids, such as the Valkyrie and Ranger used in my party, have to tread a balance of melee, ranged, and magic. As such, Intelligence comes with an extremely high opportunity cost, as does Piety and Vitality. I consider hybrids to be fighters first, and casters second (the game gives them -4 caster levels).

Classes like the Fighter and Rogue, have no such concerns, and while many would consider them boring, I consider it a breath of fresh air that they have no magic to speak of, and level the quickest in the game. They can safely max Strength and Dexterity, then Speed and Senses, without a second thought.

What a game this is, that 22 years later, such discussions are still being held. I wish there was a Wizardry 9, but we have to make the most of this Wizardry 8.

You don't *have* to do everything with one character. You can do fine with just magic. You can do fine with a lighter magic focus and some combat. But you don't need one character to be good at both.

(Also, Doubleshot Slings give you more opportunities for skill increases than Wrist Rockets, if that's a concern.)

The beauty of W8 is that you can't just get by with just one strategy. Melee only will ensure that you fall prey to enemy magic, and have to run around trying to melee enemies, while being hit with ranged attacks and magic. Ranged only will ensure that enemy magic will cripple you, while melee swarms will annihilate your party. Magic only will ensure that you can't handle bosses, enemies similar level to you, or higher level than you.
It's like a rock/paper/scissors approach. Melee allows you to survive up until the middle/late game, when enemies tend to no longer swarm you (there are exceptions), and allows you to deal with bosses quickly. Ranged allows for shootouts early on, which never stop, including Ascension Peak shootouts. Magic allows for AOE (Area of Attack), allowing for quick wipeouts of enemies, while surviving enemy casters (this falls off at AP).