Nick - You are quoting the Places category. Again, Places is a small portion of your listing. Once on the map, the Maps rules take over.
Joy - Regarding the Map Maker category guidelines, read the part about using Restaurant and then adding Mexican Restaurant as a secondary category, because Mexican Restaurant does not exist as a primary category. However, if you are a Chinese Restaurant, which is a primary category, you are to put that and nothing more. You are not to start adding extra categories like "restaurant" "restaurants" "wonton restaurant", as you already have a specific category that covers it. To quote:
Never change the category from the best possible category to a generic or less specific one.
You are to pick the most specific category, and not add other ones. If there isn't something specific due to category limitations, then it is OK to choose a generic one as the primary and then add one specific custom one (but since the primary is not even accessible within Places, this doesn't translate over that you should put the generic in there. The generic would then end up as a secondary category and would be deleted by a Maps editor). The other category boxes are for other aspects of your business, not to add more generic categories. So adding "Takeout Restaurant" is fine, as that is not automatically implied and is another aspect of the business.
The concept of category spam is explained a lot more in the guidelines for reviewers, which are not public and thus I cannot directly quote. But it stresses more that similar categories that just repeat the same thing are spam.
But the reviewers guidelines aside, the public one is where we are pointed by Google employees when they are explaining the concept. They themselves do the exact same type of edit I just did all the time, and deny edits that don't meet these requirements I'm not interpreting the guidelines, but rather I'm providing you what over a dozen Google employees have each consistently provided our group or me directly. They are also the ones that have stressed that the existing categories should be used, and exceptions should only be applied in cases such as the example where the specific category is missing. This is how it is consistently done within Maps, whether they use Map Maker or other tools. Places is the only exception, and you'll notice that they've been telling people "Sorry, your extra categories not from the list won't show" and "Sorry, in Google+ Business we won't allow these custom categories any more." It seems this is yet another place where Places is now being made to conform to the rules of Maps.
As for my choices, Plumber is a category. Plumbing Company is not a category. Thus Plumber should be the one used. Plumbing Contractor is somewhat implied by Plumber, but I looked at the Contractor categories and found all the construction ones were present except for Plumbing Company. They likely left it out since Plumber exists, but I applied a bit of leeway and added it in case general contractors are searching for subtrades. In reality it is sort of against the guidelines, but we're allowed a bit of leeway when we consider how it might be searched.
After putting Plumber as the primary and and leaving Plumbing Contractor as the acceptable categories, I removed "plumbing contractor", "Plumbing Contractors", "Plumber", "Service Plumber", "service plumber", "re pipe plumber", "plumbers", "Water Heaters", "water heaters", "drain cleaning" among others. If you can't see how that is category spam, I don't know what to tell you.
In the end, when someone has a leak, they look up "plumber". It is a valid category, and fits the "I am ..." statement, so it is the best choice. A map editor was going to change it to that, and likely would have dropped Plumbing Contractor also.