You know, this sort of boost mechanic also exists in some racing series in real life. Indycar on road courses is quite similar to boost on this game; you're given 200(?) seconds of push-to-pass and you're free to use it however you like, even after the start/restart. I think other series like DTM have also started using some sort of boost system, but I think in DTM it's limited to how many times you can use it; not sure about the duration.
People talk about Boost being gamey but don't bring up things like high level DRS very often guaranteeing an overtake on a car in front that doesn't have DRS. There's also how Boosting past a car on this game is usually a guaranteed overtake, more an indication of how overtaking on this game works and how 'gamey' it also is, with other things like how near-impossible it is to pass in wet weather past the few first laps without boost.
And there are many races on this game that are settled by close KERS battles in the last stint, which are among the most exciting things you can look forward to in the race. Disabling KERS for the last 3 laps or however removes that sort of intrigue entirely and frankly doesn't make sense considering the mechanics of iGP's race simulation I've mentioned earlier, with things like how DRS and overtaking on this game works.
So what tactics you see is a result of not just how one thing like boost works, but the result of all the things that contribute to the entire race simulation; how the cars spread apart or stick together, DRS, how overtaking and defending works, and so on. You may also be limited to seeing how the few managers in the leagues you're in race; the more you look, you'll find more and more managers who use KERS in various different ways but being able to competitive all the same. I'm not going to go into details into stuff like how saving KERS as much as possible is best in certain situations but not in others, but if you're lucky to ever get tips by veterans on this game regarding KERS, I believe all the advice you can get can be summarized in this maxim:
"Use it as much as possible, while saving as much as possible"