I have to agree on that.
As for the OP... Yes, a raw machine placed in the world, Grey are the most efficient, using the least energy per cycle, however, you'd waste more space to use them (which most people consider a non-issue) and they lack fluid inputs, meaning you can't use them for all recipes, and module slots would allow you to increase the performance of AM2 and AM3, so when moduled, the AM2 and AM3 will out perform them.