..alright, I'll tell you how I really feel
...and rant starting....NOW:
No, it can hurt by using it. Our water is perfect, you can easily make it worse. Those spreadsheets (like 5-star's 5.2 stabilizer) work great until you stick a pH meter in the wort. Anything about water that cites How to Brew as reference should be ignored. Its all based on the extremely false premise of classic brewing city water profiles. They are all either fake or irrelevant. Residual Alkalinity is nonsense. It is not science. Its closer to astrology than astronomy. You don't need a ton of Bicarb for dark beers. Ignore anything that tells you its necessary because of Dublin's water profile - Guinness is the only reason anyone ever associated Dublin and Stout and Guinness never brewed with Dublin water. You don't need any base minerals in your water. NONE. (notice that the famed waters of Pilsen have less than the recommended ion levels). The worse things you can do is add bicarbonate and sodium, if anyone ever suggests adding baking soda, walk away. Your malt will convert fine in distilled water. Everything yeast need to ferment is in the malt and it will also work fine with distilled water (once you mash with it, it is no longer distilled water)
Sorry, I didn't think it would illicit a 'trigger warning' for you.
I fully agree that the water in the lower mainland is great. wouldn't say perfect, but probably the best in the world, or damn near close to it, and it does not need toying with, as I have made a number of very good beers doing nothing with my water.
I didn't really read too much on the water aspect in How to Brew, and wasn't basing my water treatment on what that book said. I have a fellow homebrewer in my area who makes really delicious beers consistently and on a group brew day he used a little bit of chems in his beer and told me about that specific calculator. Granted he is on a well, and so his water may need a bit of treating compared to most of us. I figured it couldn't hurt to at least try it out. Sure there is always a chance I could ruin the beer, but I'm hoping it turns out a bit better. Only time will tell.
I didn't use any classic city water profiles, and wouldn't recommend anyone do so either. I spent 2.5 days looking up the water reports in Surrey and Delta, and even Vancouver where I couldn't get some of the info I needed for the calculator, for one or two points I settled on averages that I was seeing on a few reports and figured its probably the same throughout.
Why'd you bring up that sad stout? Poor poor Guinness.. I remember when I.. no actually I don't think I have ever enjoyed it.. I got in to stouts by enjoying much better stouts that it.
Just like what I've said, I would take what you said with a grain of salt as well.. pardon the pun. I've witnessed many a thread where one question is asked, and you will get every conceivable answer imaginable. Perhaps on a homebrew level you can use RO and not treat your water and beer will be fine. I mean you are pretty much preaching it now, and I'm sure you have made some fine beers. Many of the homebrewers I know that make better beers treat their water, and a number of the Local Craft Pro's do too. It is trying to go from 'fine' to 'better', but its a low low priorty, and there are more important things as homebrewers we should nail down than worrying about our water.
Maybe everything you said is correct, and it will lead to worse.. but one batch test won't break the bank here.