This right here. Was acceptable in GT5 at time as no other game on console (That I remember anyway) did swapping out with the AI (Nor allowed swapping out one AI driver for another). Now however, I would much prefer something more evolved. Less GT5 style (Apart from names) and more Motorsport Manager style in terms of stats and strategy. I know, "Eww, a Mobile Game" (Which has a PC version mind you) but you try naming a more recent console game with something similar.
I play Motorsport Manager on PC and I love it, although it is a bit bare compared to the depth Football Manager has. I've always wanted a high budget, manager mode/progression based racing game, and GT5's B Spec was the closest thing.
I liked the pure spectacle of just watching racing sometimes, with the sentiment of being a driver that you hand picked and oversaw the growth of, while actively controlling their pace and strategy.
The problem with GT5, is that for one, the AI for both you and the opposition are highly unintelligent and slow, and on top of that, the races were mostly short and so, did not involve any key strategy decision making, which is a big part of the immersion of being a driver coach/manager. But for me, GT5 was the biggest step in the right direction they took with the mode.
The carbon copy B-Spec races of GT5 highlighted the bigger issue with the GT formula and how it doesn't really work for things that try to imitate the appeal of real life racing (not to say I didn't/don't love the tried and true formula). A progression based B-Spec with a team of drivers, needs Championships/class racing, with other teams, multi-car teams, multiple races and of longer distances, too, for it to be fully realized. GT Sport and its assortment of formal, class based racing has hopefully drawn us closer to this potential ideal than ever.
I think it's a conversation that should be had, because B-Spec has massive potential and was both convenient, for when you wanted to take the load off endurance races, and fun in its own way, for when you wanted to be your own mini team principal. I don't think it's fair to call it a waste of time, or something to be removed altogether if you don't like it, but I can't tell others what to think.