having just done Rome-Venice-Rome on the ES. I completely disagree with the OP

The regional would have taken almost 2x as long. Yes it was cheaper but not worth the savings.

Also we had no problems with finding space for our bags, and I noticed people bringing bags that would only be allowed to be checked if they were flying and found space for them as well (side ways between the backs of 2 seats facing opposite directions).

At least on the Rome-Venice route there was a ES leaving 10 mins to the hr on the hr and back was @ :43. If anything there was limited regional trains running

so maybe on a short distance it may pay but I dont see it paying on the long hauls