To start of with- yes, they are rules. Some of them may be very loose, but I will go into more detail later. The reason forum rules are required is because we have to set some boundaries, otherwise people will post things and then complain when they are removed because we didn't have any set rules/guidelines on it.
I believe (if I guess correctly) you are mainly talking about double posting, which I know you like to enforce (and we do thank you for that, it makes our lives easier when others out there are instructing and guiding the community). This is a perfect example for the question you have posed above. Double posting, while it says it is against the rules, is all in all a pretty minor offence. In a lot of other forums it is a perfectly common practice, not only for adding to your ideas, but for making the thread become active again. The reason that in this community it is generally frowned upon is because we have the edit button, and that doing multiple posts is unnecessary and simply fills up the thread. Saying that, we generally don't mind if someone makes a post, then adds more to the discussion by making another just after. We would much prefer if one post was submitted, but in the end it's not the end of the world, nor on par with the severity of cursing out someone on the forums, for example. However, if someone was to spam out a thread, doing 10-15 pointless posts, then yes they would get a warning/talking to.
The same goes for "necroing". This is probably an even more minor offence- usually the only times this is done on our forums is if a new player finds a topic and wants clarification/to add their point to it, for humorous effect, or if the person genuinely has something else to add to the discussion (a great example of this is the art section, where people may occasionally add new art.) While, again, we aren't going to ban people from the forums for doing it (blarghargh that would be stoopid), it's meant more for extreme cases. For example, a troll goes through a whole lot of ancient threads and replies to them all just to be a pain.
Even though they are meant for more major cases, the rules/guidelines/whatever they are do actually matter. Without it, the world would tear apart at the seams, and the very fabric of our universe would be torn apart. AKA, the MCSG forums would turn into a fiery, necroing, double posting anarchy.
Not quite, but you get the message. To answer your last question about whether we will enforce them more in the future, the answer is no. The majority of these minor forum rule breaks are done unwittingly, and many of our community and staff are happy to remind these people that it's not how we do things around here, and we don't want to take any more action than that.