In shinto and shinshinto it's not desirable at all, same as suriage but the further back you go the more permissible it becomes. The Japanese love their brands but having said that it has to be taken on a case by case basis imho. If I found a Hankei or Horikawa Kunihiro that was mumei then the quality is going to be on a level where you can be 99.9% certain it was made by that smith for a fraction of the price.
Take these 2 swords as an example:
http://sanmei.com/contents/media/K48473_S1245_PUP_E.html
http://www.japanesesword.de/?site=sword&id=74&lang=en
¥6m and 25k Euro (About ¥3m), yet I would argue the second is a superior sword in almost every respect except it is not juyo, mumei and machi-okuri (I think the first maybe slightly suriage but not positive). It won't achieve juyo status as it is directly competing with all the signed morimitsu's. To be fair to the iesuke I don't think they're the best photo's and it's in older polish but the fact remains there is such a wealth on knowledge about Morimitsu that if the NBTHK attributes it to him it may as well be signed and it is a fantastic example.