Quote from strixflash
You cant "delete" the categories unless you are an admin. A user, however, can remove the categories he/she added to the manga, but not deleting.
About merging.. I think giving this right to every updater is a bad idea. This will cause a lot of confusion as people will try to do what they ...
Yes of course not everyone, but at least who has contributed a lot (it's a "good" editor). I pick the idea from wikipedia and quora, thinking that a (partially) crowsourced site can work very well.
About admins: great but they are few i guess, since there are tons of categories.
PS: about the link for asking "merge these categories", thanks!