I'm just not that disappointed or too surprised. You said "a little disappointing", so it's not like we're really that far apart here.
Henderson was on the street.. but we've seen multiple backs come off the couch to be productive... including CJ Anderson.
Whatever some may have considered Evans, he is a 6th round draft pick with little/no game experience. I don't see the decision as being a judgement on his potential or running ability, I see the Rams as being much more worried about Stafford's health than anything else.
QB health is absolutely paramount.