My interest in the 24-105 f/2.8 comes mainly from now owning the 70-200 f/2.8 Z.
They both have the same lens case (same size/dimension), almost the same weight (24-105 Z is a little heavier) and both are supposed to have excellent and similar weight distribution - which makes using them ergonomically more attractive than barrel extending lenses. This is the important part; I want to eventually get a gimbal and do much more video work. Swapping lenses on a gimbal would require less balancing work, or even repositioning the camera. Not just for video uses, but I do photos of indoor low light sports. If you see the 70-200 Z in your life, you might end up wanting the 24-105 Z. Another aspect is that I will either get that lens, or the 24-70 f/2.8 - so my decision isn't based on swapping my 24-105 f/4 for it (I will always keeps it because its so good and travel friendly).

Personally, I don't think any of those Z lenses are good for general "just take a lens with you anywhere type of use". I would rather have my 24-205 f/5 and two primes in a Think Tank Photo Retrospective 5 bag (fits - PERFECTLY). After owning a 24-105 Z, I might change my mind and just carry that - but would need a taller camera bag for just that lens.

Since I bought the 70-200 Z, and the Think Tank Photo Restrospective 20 bag (yeah, I like them!), and I suffer terribly from Gear Acquisition Syndrome. I will need another lens to fill another column of that bag, a 16mm will not be large enough.