Eh, I solo'd 99% of DS2 as a Sword and Board Heavy Armor fighter. I didn't fat roll but I was right at the margin.
It's really the choice: do you want to suffer immensely for a single mistake, or suffer less for multiple mistakes? Being able to throw up a shield and just "nope" an attack against some bosses is huge, like Lost Sinner. That's where the stamina pays out. You have to read attacks a lot less when you can throw that up. Armor, yeah, the damage mitigation often doesn't feel like it's worth it. But sometimes the difference of one hit can get you out of a tight spot.
The enemies that cause the most problems for people that aren't pure dodge roll are the ones that keep up a consistent stream of attacks that you can't block or take more than a couple of. I'm thinking Smelter Demon, which was the only boss fight in DS2 (other than the last one) I broke down and didn't solo. The constant rate of attacks + fire damage sucks and yeah, I can almost see a pure dodge roll character being easier there.
I didn't feel there were any boss fights like S&O from DS1 in DS2. There were almost no fights where I felt like you couldn't win due to reduced mobility. S&O fucking broke me in DS1 though, playing the same build.
TLDR: the reason for playing armor and shield is the skill threshold is lower.