TBHIMO keys were a reword to people who had them. 9/10 times people never had them, and I always picked them up. Chests that were locked were mostly 50/50 on having something good, and I can mention a good number of the uniques I found on my first playthrough were from said locked chests.
However, this is a double-sided sword, as if you make keys a neccessity it becomes more or less a regeant, and no one likes them (except for me, I was highly dissapointed when rogues lost flash powder and blinding powder).
In D2, it felt as though it was a good balance. Lets say out of every 5 regular chests, 2 locked chests would appear, and it was perfect. For people who didn't want to lug around keys, they didn't NEED to, but for those of us who decided to go the extra mile it was well worth it.
tldr, keys=awesome, but I would like to see some variety