One huge point about Ingram Spark is that Amazon's printing sucks. The covers are thin and stay curled up as soon as they're opened, the ink is cheap, and there's no QC. My Amazon covers would come in with the image off-center, and sometimes a white line around the edge where it didn't line up.
Ingram Spark uses heavier coverstock with a crease down the gutter, heavier paper (you have to rework your spine size because even with the same interior file, the Ingram Spark book will have a thicker spine), and the ink appears to be of much higher quality so the text comes out crisp and the covers pop. Plus, Ingram Spark allows for hardcover printing.
When I put a KDP print proof next to an IS proof, the KDP proof makes me feel like I need to adjust my glasses.
KDP hardcopies look like they were made by an amateur. That's just what it comes down to. Compared to a professional offering from a major publisher, they look like they were printed and die-cut at Office Depot.
A properly set-up IS hardcopy, with a professional cover and typesetting, is virtually indistinguishable from a Big Five offering, aside from things like embossed covers, metallic print, etc. I just did a signing at B&N, and the store staff were admiring my hardcovers and asking which Big Five publisher Oxblood Books (my publishing company) was an imprint of. The staff.
Now, this is just me, and this is the market I play in. It may not be what everybody needs.
The other piece of this is that with IS you can set your discounts and returns, so you can get your books carried in physical stores.
Readers who buy perfectbound and hardcover copies (as opposed to trade paperbacks) do it because they don't just want something to read; they want an interactive work of art. Give them their money's worth.
