I think what happens is that people read that distilled and RO are missing the minerals that are needed and either miss or misunderstand the part where it applies to all grain, then repeat what they've read (or what they thought they read). Others read that and repeat it and the misinformation spreads.

Funny thing about the water...when I do extract batches I use tap water HOWEVER when I do my normal AG batches I throw in a little bit of Gypsum with that same tap water. My earlier AG batches were a little blaw then did a read up on the water and discovered this ingredient that has made a huge difference as all the flavors of the beer were enhanced when I used it. Just that one mineral addition improved my brewing. However I don't have the same water that they have in Milwaukee,St.Louis,Chico CA,or Kalamazoo MI. Know your water source before acting upon it.

good points, milltime.

if anyone REALLY wants to start messing with their water chemistry, especially the all-grain brewers, most municipalities have a water report on file with dissolved mineral and chemical make-up...

Even simply running your tap water through a home filtration system (Pur, Brita, etc) can often make an improvement.

I had my tap water tested and it's perfect for making stouts. Unfortunately I'm not a big stout fan so I often compromise by using distilled to cut the mineral content of my water. The lighter the beer I'm brewing the more distilled water I use. But, all this applies to me brewing all grain.