First let me say that my level of expertise on the subject of microphone restoration is nowhere near that of many others on this board, including of course Klaus.
That being said, I'd like to point that that in my limited experience, there are a number of factors that contribute to whether or not a capsule can be cleaned.
First, What kind of dirt is it? Some things, such as dust and saliva, will come off fairly readily with distilled water and a practiced hand. Other substances don't come off so easily.
Second, has the capsule been cleaned before? If so, how carefully was the job done? I've seen capsules that were 20 years old that had been cleaned several times and showed signs of wear - i.e. the gold deposit was worn or flaking, etc. In these cases, cleaning may damage them further or not be possible at all.
The statement that "why bother cleaning it, it will be the same in 3 months" seems quite far off, to me. If you are using your microphones in a dust-free environment (is there such a thing? <g>) and with a pop screen, etc. then you may go for several years without needing a cleaning.
One thing that I've seen done is to simply turn the capsule around, so that the side not normally facing the singer is now the front. Usually this side of the capsule is far less dirty, and since most people rely on the cardioid pickup pattern for 90% of their work, this method is viable.
The bottom line, I think, is that most often capsules can be cleaned with a minimum of fuss. This is depending of course on the above issues.
Regards,
-Karl