I will address each of your points because they are decent points but none of them amount to much of an argument. I have been a member of other forums before where this same sort of thing happened and the members felt entitled to free content.
"People are used to getting content for free on the internet."
Tough. Just because a lot of stuff on the internet is free does not entitle you to free content on a forum or digital magazine. Most of the 'free content' on the internet is only free because of the advertising.
"The member content of BPL is variable and declining."
Says who? I hear nothing but great things about the members section and all I see around here is article after article on varying subjects coming out. I also see the staff members constantly asking what people want to see more of and I assume incorporating that into the members section. Also this is by far the best light backpacking site I have found and I assume the same applies to you otherwise you wouldn't hang here complaining you would just go someplace else.
"If a non-member user puts up 2,000 helpful, content-rich posts, are they a freeloader (or are they, instead, creating valuable "content" for free)?"
Absolutely not. If a forum user posts information on a known, public forum/message board they are offering that information for free. BPL does not monetize their content. If they took user content and put it in a book and sold it then your point would hold some water. If you think your information/posts have value I suggest you publish them not post them in a free, public forum.
Additionally I think it is plainly obvious that this site is an online magazine that has some free forums. It has always been clear to me that the forums are basically a teaser to convert users into members.
"Restricting selling on Gear Swap might reduce the benefit to all."
This is the only valid argument. I would simply say though that I think the BPL staff probably considered this point before hand.
"As attempts (scattershot, poorly communicated) to monetize BPL occur (e.g. MLife, user restictions, ads) what will the effect be on usage, the vibe of the place, solvency, etc.? "
What will the effect be on usage if people who are not paying members pledge to not pay to post in the classified section? I would say zero effect on BPL bottom line.
"What (if any) is the vision for BPL and a strategy to implement that vision? "
Is that really any of your business? Do you ask these same questions of other businesses that you patronize? Would you walk up, in person, to your favorite restaurant owner and demand to know what their vision and strategy is? Better yet, what if you never even bought anything in that restaurant, all you did was went in, ordered water and used their free wifi? Then they start charging $.30 cents for the water to cover the cost of the styrofoam cup and you demand to know how they think charging you for water while you get free wifi is going to affect their monetizing strategy, their bottom line, and the vibe of their restaurant?
"Should everyone pay for any use of BPL, including free forums, in light of issues above (and many others), and if they don't, is their input to be marginalized or wholly discounted? Etc. etc."
Yes, the input of non paying members should be marginalized as they are not paying and therefore are not customers they are free loading. They are using the free wifi but not buying the food so they should zip it and enjoy the free wifi.