Don't listen to them.
Higher resolutions look great on canvas. You can see the difference. Especially in color saturation and density.
I run 1440x1440 32 pass on my JV33 (CMYKLcLm) and it looks phenomenal!
Using the higher resolution offers a 60% increase in available color gamut. Using the higher number of passes allows you to lay down more ink.
Sure it's slow. You simply cannot argue with the results though. If you want brown washed out reds go with 720 8-pass. If you want to be able to reproduce deep rich luscious colors, bump up the resolution. See the attached pics for proof of concept. It helps to create your own profiles in-house as well. Not only do you get more accurate colors, but you can usually increase the available gamut over the canned profiles.