My extremely limited understanding is that you don't want to use softened water for brewing. You actually WANT the Calcium and Magnesium in your brewing water and a softener removes those elements and puts Sodium in their place (actually it adds 2 Sodium atoms for each Calcium or Magnesium atom it removes). If your water is not too hard then this won't be too much Sodium and you can get away with it...but usually the whole point of a softener is because the water is excessively hard (which means a LOT of sodium in your beer...and no Calcium or Magnesium).
Generally speaking, dark brews like Porters and Stouts are good for hard water and light brews are not. Reason being that Porters/Stouts use a decent amount of dark grains...dark grains acidify the mash so you need more alakaline water to get the correct mash pH. Alkalinity and hardness are two different things but if your water is hard it's usually also alkaline.
You really need the whole water analysis profile though. Brewers are usually more interested in mash pH and mash pH is dependant on water hardness, water alkalinity, and the grain bill (among others but for simplicity's sake...). Hardness actually decreases the mash pH because the Calcium and Magnesium (to a lesser degree than Calcium) react with phytin in the grain to create hydrogen ions which neutralize alkalinity. Alkalinity increases the mash pH. So it's really a balancing act between hardness and alkalinity to achieve your residual alkalinity
, and residual alkalinity is what brewers are more interested in (because it directly relates to mash pH).
Read chapter 15 in Palmer's How To Brew
. Hope that helped.