First notice I said copper based thread coumpond not loctite or a thread locking compound. What is the copper based thread compound, grease with copper flakes in it. There are other similar compounds that use other metals but copper is the most comon for most uses but you can get lead, zinc, and if you happen to have a atomic power plant nickel is popular. Why is that better the grease? Because the microscopic copper flakes are compressed between the threads and form layers of soft flakes between the harder metals of the threads. One of the problems of just using grease is that the grease is extruded as the threads are compressed together. Someone might jump in and say the grease keeps the balls in bearings from touching the races so it should not be extruded. That's true in the dynamic nature of a bearing but is not the case in the static nature of a threaded connection. This copper does several things, one of which is to make a metal to metal seal the keeps water out. A much stronger seal then grease, The most important feature is that when you remove the thread the compressed flakes shear rather then the metal of the threads galling on each other. You mention that some books recomend using grease, true grease is better then nothing as it fills the roots of the threads and can help prevent water getting in but the thread compounds do that and more. You mentioned plumbers tape. Great, it works just like the copper in that it forms a microscopic layer between the surfaces of the threads that will shear and fills the roots. To tell the truth I have also used it on the bottom bracket of my bike, mainly because it's a little neater. I have also used teflon tape on seat rails to quiet squeaks. Bottom brackets and most other threaded connection will do better with the copper based thread compounds rather then grease. There are exceptions suchs as bolts that must resist a rotational dynamic fouce such as suspension pivot bolts here the manufacturer will often call for blue loctite.