I just want to differentiate between minerals added to the mash and minerals added to the boil. If you're happy with your mash water mineral content, you could just add some minerals to the boil and that would result things like increased perceived bitterness and benefits for yeast/fermentation.
Concerning the mash, Noonan says that calcium "stimulates enzyme activity and improves protein digestion, stablizes the alpha-amylase, helps gelatinize starch, and improves lauter runoff." But how much is needed and how much is contributed by the grain itself? Calcium, next to potassium, is a major mineral constituent in barley.
My water (from a well, unfiltered/untreated), is extremely soft and low in minerals. Its Ca content is 2ppm, its total hardness (CaCO3) is 9ppm, and its total alkalinity (CaCO3) is 7ppm. Most other minerals are low to nil as well and the pH is 5.7. I don't treat my mash water at all and get 89-93% efficiency and don't have any other issues related to what Noonan states.
I swear I've read somewhere (but can't find it right now) that barley malt itself will contribute calcium to the mash. I find this interesting being that my water is so low in calcium, yet I don't seem to suffer any ill effects to the mash. I just add a little gypsum to the boil, less for some beer styles (German lagers, e.g.), more for darker/higher IBU beers (British styles). I mainly do this for the yeast, but also to help with flavor in British styles.