Maybe part of the problem is the term "spam". It has a very strong negative association, and if you are accused of "spamming" if all you were trying to do is show off your hard work, I can imagine you get a little upset. Maybe "advertising" is a little more neutral term.
With that said, when you're enforcing board rules, the intention
behind a post may be very important to the poster, but is something you, as a board maintainer, can't really take into account. If someone "advertises" their board by posting a link, they are not allowed to. If someone "shows off" by posting a link, they should be allowed to? Problem is, how do you know the difference? The first guy could easily say, "hey, just showing off
Anyway, from a practical point of view, I think there is an enormous amount of horrible sites out there. If I could take a swipe at how much sites would actually be of interest to me, because of a particularly nice design, or some clever added features (either by installing mods or homegrown), I'd guess it'd be 1% of all phpBB boards out there. The rest would either be standard installations, or horribly overbloated installations with every mod under the sun installed and with some horrible homegrown style or some third-party downloaded style (I can see those in the styles demo). Now, I think it is fair to assume that a "show-off links directory" or forum would have about the same signal-noise-ratio. Maybe a bit higher, because phpbb.com would naturally attract more "involved" people. If the amount of "signal" would be 5 times higher than in the world as a whole, there would still be 95% of stuff I would not be in the least bit interested in. I don't think anyone should invest time in maintaining something like that.
Maybe an idea could be to run some kind of "Featured sites" series on the phpBB.com blog. People can submit their sites and someone picks out the particularly nice ones and writes an article about it. Feel free, anyone, to run with that idea, because I personally do not
have the time for that