As stated above, stars are not worthless and worth shooting for, but promoting is more important in the long run. I have 1-2 star heroes fully promoted that are just plain boss compared to 4 stars of others.
Some facts:
Evolving only increases basic stats, this does improve advanced stats like attack dmg, crit, hp, etc but only by flat amount. Helpful, no denying that, but not game breaking.
Promoting:
You gain all?(some?) of the stats from the items on the character during promotion AND when you cross a color tier you gain a new skill, which I literally cannot express how valuable that is in this game. obviously some skills are better than others but in general the principle holds for all heroes.
Face a 4 star blue vs. a 3 star purple of the same hero and watch
That's 1 "tier" difference for each side and you'll see an amazing gap. This game is far from perfect, but for whatever reason(poop for a long time?) I like these stupid little "half attention" iphone games such as this and brave frontier.
Bottom line, promote all your heroes the best you can and max their skills. Prioritize as your team and options permit.