I agree on those points, but I think another (secondary) reason in the mix for some manufacturers is to develop new artist relations and have existing artists " show the flag".
This helps create and continue a "buzz". I'm not saying this aspect is the main reason they attend but I think given the coverage by media, online sites and forums, and having the opportunity to include NAMM happenings in their advertising to the public, NAMM adds some value for marketing generally. Maybe now especially with the internet.
NAMM also creates a consumer focus and some anticipation amongst a growing number of consumers...it does create some buzz and excitement beyond the seed of manufacturers just selling to retailers IMO. But I agree not every company wants or needs that though as you say.
+1 in general. Given the cost, not sure that NAMM is the best venue any more for that, but those bass hangs, like the one in February that Jimmy is attending and sponsoring (posted above in this thread), is key for the type of thing you are talking about, and is, I assume, pretty cost efficient.
Edit: Also +1 that Jimmy's massive buzz on Talkbass was (if I remember correctly) really kicked off by all of us seeing that beautiful array of instruments 5 or 6 years ago (or whenever) when the NAMM pictures were posted. Roger Sadowsky, Mesa, etc. all put their time in at NAMM, and IMO (just like Jimmy) were smart enough to know when the cost and hassle resulted in that 'diminishing returns' point. +1 that it can really kick off a brand if all the planets align.



