Hmmm... well, I know quite a few people heatsink some chips in older Commodores (64, 128, etc) as some chips on those werent originally heatsinked. Commodore 8 bits and Amigas were probably not designed with the idea that people would be using them for decades, since Commodore wanted customers to buy the newer machines every few years. Heatsinks may not be required for that kind of use, but as these machines age, its probably a good idea to do whatever you reasonably can to try and keep them working. While the Amiga chips may not get hot enough to need heatsinks, its not going to hurt anything to add them, just in case. I also wouldnt consider "if they were needed they would've been added originally" to be a reliable guideline... I've seen plenty of Amiga accelerators with 68030+ CPUs that get nuclear hot yet do not come with heat sinks.
Heatsinks are cheap and easy to get. Replacement Amiga chips aren't, since most of the time you have to buy entire systems to get them. And some day, the supply of working parts that people are willing to sell will run out.
