It's not just about joining a signature campaign either, the primary concern is "controlled" SPAM from Members trying to achieve the next "Rank".
Which is perpetuated by Signature Campaigns. Other than making yourself look better, there are little to no reasons for you to rank up other than to achieve higher pay in a Signature Campaign. While this is not your reasoning, I expect it is the main reasoning for the majority of members.
Something simple as providing your BTC Address or taking a look at the date you joined a Mining Pool are just two fairly simple ways one could provide some background that would confirm they are not a "Newbie" (if they choose).
Please remember that your rank on this forum is just that, your rank on
this forum. For a Legendary member my technical understanding of Bitcoin and other topics is very limited, however that is not relevant when evaluating my status on the forum. Whether this is correct is debatable, however it is how it currently works and I don't expect that to change.
Addressing your ideas, they would require unneeded intervention from Staff to be applied and would be open for significant abuse.
Staff are already busy with several other forum needs, furthering this just so that a user doesn't have the word "Newbie" on their profile seems rather vain. Does it not?
In addition, provided that the incentive for high ranked accounts (signature payments) was still there, it would be easier than ever to farm up accounts by adding a unique Bitcoin address and linking a mining account to them to rank them up. This would
not be good for the forum; it would make spam - a problem you identified in the first part of your post - easier, more lucrative and harder to remove than ever before. Surely you can understand the negative implications that would have here.
I'm confident this community could come up with one if asked to take a serious look into implementing some sort of automated function to determine ones level of experience with digital currency..
Perhaps, though it doesn't seem like a good use of the communities' power or creativity considering how little it is relevant IMO. If your rank on the forum bothers you so much, you can either
purchase Donator to change it manually, or post in the topics you are interested in to have it rise naturally. Personally, it seems like a waste of time and effort to try and implement anything else.