I actually think they were a great addition to the Pokemon series. Albeit a unexpected one, I didn't see it coming at all (I assumed the fact you could buy potions, full restores, and with the addition of berries too, would see the end of "Outside PC" healing, as it were), but they're definitely an addition I'm not complaining about.
I didn't find it was the healing due to being weak that was the benefit for me - More so it was the fact that I could get some training due mid-cave/route, something that was rather difficult in other Pokemon games due to the significant lack of Ethers, and the amount of PP lost while trying to train. Thanks to these Doctors/Nurses I was able to keep my Pokemon's PP to a sufficient level that I could actually train with them.
Cool topic. :)