Unfortunately, B12 blood serum test tends to be the only test many doctors run to check for B12 deficiency. As you have stated, this test is very unreliable. The low end of reference ranges in the USA is way too low. In Japan, 500 is considered to be the low end of normal. Also, if your body is not converting B12 to an usable form, your blood serum could be falsely elevated. There are doctors who will also check homocysteine and MMA levels. Elevations in these, indicate a B12 problem even if B12 serum is normal. Even then, this is still an inadequate method.
I think many have insufficient levels of B12 and it very easy to correct with oral supplementation. In addition, there are active forms which work better than the common cyno form found in most vitamins and the B12 injections.
Once you correct the deficiency, it can take quite a while for your body to repair things. And then, sometimes repair is not possible if the deficiency has been going on for too long. That doesn't mean it's not helpful for managing the disease. It's alway good to have your nutrition at optimal levels if you can.
Here's a link on B12. It was written back in 2003 but is still useful.
http://www.aafp.org/afp/2003/0301/p979.html