Another advantage of the toile : I keep it as reference. So if there is something strange happening on the baste, I can look at the toile for comparison and check if I forgot something or made a mistake.
I also like to try different ways of cutting a coat, so a toile is an inexpensive way of taking risks. If I'm not satisfied, I can always come back to a tested and approved method.
Even when I make a toile, I still make a skeleton baste if I'm not confident with a specific cloth. Sometimes the toile has no collar nor sleeve, so the skeleton baste is very useful.
Toile allows me to design, also, as we (the customer and I) can draw on it easily the pockets, the amount of cutway ("opened quarters")... Customer has a greater feel of "bespoke", as the design is interactive, than when I cut directly on cloth. On the other hand, it is sometimes hard for the customer to imagine the finished coat when seeing a toile.
JC : I like to use plaid when I can, too, for the same reasons as you. I got the trick from a dressmaker who uses gingham coton fabric instead of usual muslin. If I find no plaid at cheap price, I try to find something with vertical effect, like stripes or herringbone, it is still usefull.
I do not know what "last year style" means, though. That is not my world