As far as
SEO ranking and optimization, these URLs are going to be ignored by the search engines - they have no content, only redirecting to your main site, and they will probably have no links. There is nothing for search engines to rank them on or to show in the results pages, so you wouldn't get a benefit of having these domains indexed. I am not sure if you would be penalized for it, but it wouldn't really give a benefit that I can see.