p. 183 "multiple simultaneous blasts" kicks in when using semi-automatic bursts with grenade launchers. It looks like this rule was specifically made for the SAB, because the explosions have to occur on the same initiative score. Different caracters throwing multiple grenades will never(<1% of cases) have to use the simultaneous blasts rule.
Actually, with triggering their detonation wirelessly, you can pre-throw several grenades and then trigger them to detonate. In a recent run, I placed 3 flash bangs near an entrance. Me and them team extracted our target via stealth and Improved Invisibility. Once we cleared the blast radius, I detonated the flash bangs on the group that came to investigate the commotion within the prison chamber. The 'boss' and five of his goons ended up having a really bad day while we continued making our escape through the complex.
Also, imagine a scenario where you have some heavy opposition against a secured door. You and your team also have full cover. You throw over toward their position at a penalty, but even if you screw it, scatter rules aren't that bad, especially with hand tossed ordinance. By the time you have your second or third grenade over (likely 2nd ini pass), you can detonate them all, assuming they haven't fallen back through the door or started lobbing grenades back. This is completely separate from anything else your team does (suppressive fire, conjuring a f6 spirit to keep'em occupied, then having it dematerialize before you detonate, etc.)