Yes Bopp! it was great, though bringing it back would require some adjustments to it and sparks (I think the sharing health revive itself would benefit from some timer so it is not possible to just run around and revive teammates without the need to stop at all like it was back then which was a little bit op). Anyway, I think devs know better how to balance it out.
I miss this system a lot (even more than I miss falling on my butt from the Iron Slug charge /gi), and I agree, it is completely different psychologically: not having this feature makes solo runs more preferable overall (solo runs are already pretty good due to crowd control) and eventually this efficient solo grind leads to burnout and quitting. An average grind run becomes a fun adventure when there is teammates who are actually able to take care of each other.
You're talking about reviving a fallen comrade by sharing half of your health with them? I agree, that it was a nice teamwork dynamic.
It was removed in the July 2013 mega-update. It was sort of replaced by sharing Sparks of Life, but that is psychologically different, because Sparks of Life can be hoarded and therefore must be hoarded. ;)