Why should I upgrade to Thorium Reactors?

The upgrade is togglable, meaning you can turn it off and on with no penalty. There are a number of reasons you should want to turn it on for some time:

• to supply more electricity to temporarily turn on some energy-expensive stuff, like heatsinks, or accelerators (more pocket rifts).
• to combat the electricity cuts by some calendar cycles (Dune ⍫ and Terminus ⍝) - this is only if you have the Numerology metaphysics upgrade.
• once you get past a certain point, the bonus you will get from it will be large enough to supply accelerators to get enough uranium, though I don't expect this to be possible before quite a few resets for paragon bonuses and metaphysics.
• in case your net electricity production is just below zero, you can toggle it on every once in a while to use the excess thorium and get a minor production bonus.

Thorium can be crafted from Uranium, which can be generated from Accelerators or planet crackers.

If this upgrade doesn't do anything for you now, that's fine. Don't use it now. It'll be useful in the future.

Not all upgrades are immediately useful, and late game has many cases of this.