[FranOntanaya]

It's still boosting the science profile of the mission to use it as test bed for Americium. There's a lot of science to be done in the Kuiper belt and further out where longevity may matter eventually simply because we wouldn't be sending repeat missions to each dwarf planet out there anytime soon.

[jjk166]

So instead of 10% more of the high power density material, you want to use 500% more of the low power density material?

[Retric]

Weight of a single component isn’t the only consideration. Having less heat output on the launch pad is a real benefit. It may take a 200 year mission for the weight to balance out, but having less variation in output is a benefit on day 0.

Further there’s many reasons why someone managing a project may say no to 10% extra plutonium even if the project could benefit from a longer lasting power supply.