Taps are worthless to me, other maybe selling it to an enemy planet and force-spawn a Pather cell on their planet. They give +8 to pather interest (and Pathers only need +7 to spawn a cell on a size 4+ planet). Also, to meet demand, I need catalytic core and possibly alpha core to boost production. Maybe AI core to reduce demand too, if that is not bugged.
It only protects against faction expeditions, which are rare and irrelevant in threat compared to the pather doomsday clock.
That is what I use would cryoarithmetic engine for, faction expeditions. Since I never use AI cores (except in Automated Ships) and only use one item per world, I never get Pather cells on any of my worlds. With Hostile Activity at zero, faction expeditions are the only thing I need to worry about. In this release, they have been rare and small, but after about 15 years, they started to grow bigger (sending two fleets instead of one), and I am not sure one world systems are expedition proof long-term.
I would consider an engine for a one-world system with a hot world, if I had nothing more important to use. That being said, I have a hard time justifying it. I already use two forges and a synchrotron on three of my five worlds (which I need to feed Military Base or High Command, the big resource hogs), so that leaves two worlds left for other items. The fullerine spool (from Eventide) is nice, so that probably leaves one world left for whatever random item I find.
Faction expeditions in the past are why I am encouraged to put everything in one system so that they can defend themselves without player intervention (i.e., babysitting). If not for that, I would colonize wide and have one or two colonies per system, not three or more in one or two systems.
I do agree drone replicator is worthless (aside as possible tribute to Kanta), when it gives +4 to Pather interest. For it to be useful, the battlestation/star fortress needs to be knocked out before enemy expedition can interact with ground defenses. Losing the battlestation for months hurts since it provides a huge chunk of stability. Thus, better to buff fleets with more bases in-system and upgrade the station when needed to stop the fleets before they blow it up.