The trouble with older computers is that over time, especially if a fair amount of heat is involved, the electrolytic capacitors can degrade and stop working. A tell-tale sign is that they start bulging at the top of the capacitor if it is soldered upright in the mainboard. That is why so many mainboards now boast 'Japanese solid capacitors'.

While you can replace the capacitors if done carefully, it is very nice to know that if the computer in a RFM does die, you can install a replacement.

It is not essential to have a Nucore kit, but if I ever bought a RFM I would most likely buy a Nucore kit as well as insurance. However, if you don't want to spend an extra 1K now, you can just wait and get the Nucore kit later.