I recommend cleaning the kettle after every brew or series of brews. This was discussed in another post. Basically, if I brew once in one week, I will clean the day after the brew. If I brew 4 times in one week, I will clean on the 5th day. To leave it dirty and out of use seems like an open invitation. Also, what you may be seeing in other kettles is not dirt, but oxidation if not stainless. If you have a stainless steel, steam jacketed kettle then you can make it shine. Otherwise I think it normal to have some discoloration. I'd imagine a direct fired kettle will also look "dirty" even when clean although I've never brewed on one so I'm not sure.