I would say this is part of the game. Each and every shot is only worth a lot in the right moment. I.e. the subway shot is worth nothing until a sim card is lit. But if you earn a sim card with it, it is one of the most valuable shot. Same with the multiball. It is worth allmost nothing (but fun to reach for newbies), unless you have a suitable mode started or a lot of sim cards to collect. So it is part of the strategy to start the multiball at the right time. I think this is a nice concept compared to other games where the mutliball is giving high points no matter what you prepared. I really like this concept as it adds a strategic value to it.
BUT... I agree that the forces multiballs can be annoying. Of course this can be considered as part of the risk - I cannot avoid the multiball as long as I want if I drain a ball while a multiball is waiting. I also should wait with the locking of the balls until I have sim cards stacked to avoid it start with the next mode. So it is kind of oppisite to other games, where I try to lock balls as soon as possible to start as many mutliballs as I can to get high points, because I have to avoid the locking as long as I think I can stack more value to the multiball and make sure I am not forced into a worthless multiball.
So in the end I am not totally happy with the way it is, now, but I can imagine that a free choice of delaying / starting a multiball would make it very boring.