The thing is its only a 'dps perspective' where this is true: Its certainly a long, long grind (with many past lives) to bring instakills up to the level you talk about, and that DPS is a lot more limited when it isn't supported with baubles, eardwellers, other mana-regen gear, or sp-reduction gear. Since casters don't really do DPS (but rather do a set total of damage with their whole mana pool), endurance extention -is- a huge deal. When I upgraded my savant to an epic cloak of the zephyr and went from running most epic content with a lv 16 ornamental dagger in my hand (for the major lightning lore) to running with the staff of the petitioner (for the sp reduction, since I've also got epic inner sight), the difference in my soloing was gigantic: I now reach shrines I used to reach dry and kill the last mob with a SLA when I've still got 400 mana left, I frequently don't have to use my bauble or other regeneration clickies, and etc.
My character's performance was drastically increased by that item, but not for reasons that have anything to do with its theoretical improvement of my DPS (via the 12% crit chance). The improvement to my instakill, mass hold (to nuke the held stuff) and endurance is what changed, and even that simple upgrade on a character who had pretty much everything else was extremely noticable. It was certainly more noticable than any ravager set I've ever completed, or any claw set, all on its own.
Casters improve in other ways. It takes great play to play a very badly equipped caster and be an effective soloist in content that is any kind of a challenge, and its difficult to play them and beat melees in killcounts since the sp pools are bad and the dcs are not so good.
Its no surprise that extremely equipped instakillers are great at trash mobs. Those characters (palemasters and divines) require a lot of trs and actually aren't that good at dpsing: Even wizards running 2 dots don't eclipse high-end melees, and if they do any other kind of nuking their damage potential dies quickly. Sorcerors can blow melees away, but they don't cart around 45 dc necro spells.
A mix of those is obviously extremely powerful, but so is a mix of one of each, a healing bard and a bunch of great melees.
There have always been classes that were disproportionately better at record-set speedrun situations. Remember how the fastest shroud ever was a group of 11 monks to touch the portals as fast as possible? Paladins have always been very bad at speedrunning because so much of their dps is lost if they skip shrines (2 mana buffs, and divine mights).
If you can do 15m shrouds with all melee groups, it doesnt matter if an all caster group does it in 12 or 13. They're all really, really fast.