"It's like that old myth that you should change shampoo brands every couple of months as your hair "gets used" to the stuff and it doesn't work properly. Lol!"
Actually, that's not necessarily a myth!
Over the last few years, most of our popular shampoos and conditioners have been reformulated with new, space-age polymers and silicones that really can build up and cause other problems if they are used without interruption.
I have thick, coarse, unruly hair the consistency of Brillo...in order to get it the way I want it, I generally use products ordinarily targeted for the African-American market, and they tend to have oiler, heavier ingredients that build up.
So, once a week, I shampoo with Suave Clarifying, White Rain's transparent shampoo, etc. to take away the buildup, so it doesn't get to a point where my hair is flat, dull, and lifeless (like me, on Monday morning!)
As far as detergents go, the only case I would definitely think it would be imperative to switch off brands is if you used a product like "Tide with a Touch of Downy" in America, or "Bold" in the UK, along with a seperate fabric softener in the rinse cycle. Lots of people do that (I used to as well) but the softening agents in the detergent, more than anything else, tend to build up over time, and I think the follow up softener or dryer sheets will leave a coating that "seals in" those agents, or at least that's how I'd imagine it.
Buying the new detergents that come out is sort of a cheap hobby for me....once in a while, I'll find a new product that is the best thing I've ever used....most times, I'll try it, not like it, and give it away to someone who needs it more than I do, so what is lost?
I don't think most people are like that...they are still brand loyal and toss their favorite familiar brand name into the cart, only to discover it's "new and improved", with no resemblance to the product they loved!