Disagree, though I understand your point. You don't buy/not buy a lens solely for max aperture reasons. That's just one consideration.
- Both 24 primes you referenced will be sharper than the kit zooms, esp. on the more wide open end, say f/2.8 - f/4 or so.
- The pancake is as big as a minute with only vignetting, focus by wire and STM focusing speed as it's only limitations. Just like the EF 40mm, it's shockingly sharp for its size.
- The 24 f/2.8 IS is in another league entirely. Sharpness equivalent to the 24L II. It's a fantastic lens with modern fast USM, internal focusing, great build quality and of course the IS. That's effectively a "slow" 24L prime without weather sealing, and I honestly rate it as good a value at $549 as the pancake is at $149.
Kit lenses are great, but both of the 24 primes you referenced are clear upgrades over kit glass to me.
To be fair, I rarely used the 24mm IS. I think I used it a handful of times and each one was basically me forcing myself to use it.
But... take a look at the PhotoZone resolution charts for the 18-55 IS STM and the EF-S 24mm f/2.8 STM. They're not very different at all.
EF-S 24mm: http://www.photozone.de/canon-eos/889-canon_24_28stm?start=1
Check out the 18-55 resolution chart at 18mm and 24mm (below). Again, neither are at f/2.8. But at 18mm and 24mm (f/3.5 and f/4 respectively), it resolves almost identical lp/ph in all 3 measured areas. So, it's probably safe to say that 23mm at f/3.5 is in that neighborhood.
Now, check out the EF-S 24mm f/2.8 STM.
It's basically the same as the 18-55 STM (it resolves about 3% more in the center than the 18-55 STM). There are some minor differences, but likely nothing that would show up in the real world. So, the assertion that the EF-S 24mm f/2.8 STM is a "clear upgrade" over the 18-55 IS STM kit lens is not true at all. You're exchanging a loss of 2/3 of a stop of light (at 23mm) to gain 4 stops of IS plus zoom capabilities for 2/3 stop of light gain plus a smaller size and lower weight... and $150 gone. Again... IF you already have the 18-55 IS STM.
So again, my recommendation is - if you already have the 18-55 STM, don't bother with the EF-S 24mm f/2.8 STM. Yes it's smaller and lighter and 2/3 of a stop faster, but the kit zoom isn't heavy and it isn't big by any means. Obviously if you NEED or really want the small size and or the extra 2/3 of a stop of light, then yes... go for it for $150. But don't buy it just to buy it if you already have the 18-55 STM.
I have no doubts the EF 24mm f/2.8 IS performs even better. After all, it is a full frame lens and we're talking about using it on a crop sensor camera (PZ hasn't tested it). But we're also talking about investing $550 for it too... and is it $550 better? Maybe... if "excellent" in the center and "very good" everywhere else isn't already enough.