Yes and no.
Consider that, for a start, it's probably not all that much more than what people normally spend on magic, just concentrated in two models: Wandering/Focus, Channel x 3, Reservoir, Soul of Stone + Tetto + Skink Priest with a Dispel Scroll. That's already pushing 800 points, and you can happily get two Slann in at 400 points or less each.
The good thing about having two of them is that you can distribute your General and BSB so you aren't totally all eggs in one basket, and this allows you to be a bit more risky with casting. Because miscasting isn't as scary as it is with your Slann, you can actually skimp on some upgrades (ie Stone).
The other point is flexibility. Go Loremaster High and Wandering Deliberations and you have a ton of options, but it gets pricey. I was running just 4 on Light and 4 on High and I found that this gave me all the utility I needed, which was considerable. What this means is that you can actually skip Wandering Deb or Focus of Mystery as well, and your Slann might come in at less. Finally, the ability to swap spells between Slann so that you are always in range for what you need should not be underestimated - if only you can remember it!
For instance, I had just the two Slann, one with x3 Channel and BSB, the other Reservoir and Dispel Scroll, for some 700 points.
That being said, I agree that it is not worth it with Kroak. Kroak's power is from being able to spam his spell each phase. If you are doing that, it leaves you precious dice left over for anything else. Some sort of back up caster is arguably worthwhile for combat buffs if you are playing it so that his spell doesn't affect combat (Tetto is perfect for this), but a second Slann alongside him isn't going to see any benefit.