Why would someone do an SWG when chlorine injector systems like that are available? The self contained dosing system you linked to is maybe half or a third the cost of an SWG and I have to imagine a handful of 5 gallon jugs of chlorine a year is the same or cheaper than a salt cell every few years. It seems like a no brainer to me, but I'm new at all this and don't know any better.
Because it is easy.
Just to go back and answer some of your questions.
1) There is a lot of mis-information about pool water chemistry out there. Some of it is because of old legal requirements for certain pool types (such as no CYA in public pools), some of it is 'folklore' that has somehow become accepted as true when it is not (green hair and chlorine) and some of it is pure marketing (pool store testing that makes pool care seem complicated). People will swear by this mis-information.
2) Your pool store employees are not pool experts. No more so than you would expect the person at your grocery store to be a master chef, or the people at Macy's to be fashion designers (or sadly, the people at Home Depot to have any clue how to use the stuff they sell). They get rudimentary training and most of the time are working off of a flowchart or manual with no real understanding of what they are recommending.
3) The people on this site gain nothing from you following these methods, except the satisfaction of knowing that they have helped somebody. This forum has pool industry people on it, as well as a few chemists that can really get into the science behind the recommendations. There are also dozens (if not hundreds) of run of the mill pool owners on this site who practice the methods used here, and have crystal clear pools with low costs to maintain to show for it.
4) Just like with most things in life, there are different ways to do things, and different cost / benefits. Me, I just went to a SWG and I am very happy with it. I used to use bleach. Was bleach difficult - no not really. I added a quart a day before work while my filter was running. Not to difficult, until I have to travel for work, or it is raining, or any other number of situations arise. I could have gone with liquid chlorine and an injection pump, but then I would have had to still buy chlorine, and I would have to add automation. With salt, I have my system set, and it does its thing 2x a day. Every so often I test my water to make sure things are still OK. Even if it comes out costing me $60 a year over the life of the cell, it is still well worth it to me. It's not about trying to spend as little as possible. It's about not spending money when there is no benefit. Pool stores often cause you to do that.
5) Pool stores do have their place. The Leslie's by me does have a person who is quite knowledgeable about pump / filter hardware. If I have a cracked part, or something that is wearing out, I bring it in and he finds me a replacement. I also run by the store to buy pool lube whenever I am working on something with O-rings or seals. like when I break down, clean, and store my filter at the end of the season.
-dave