Perhaps it struck us as trivial, or maybe nice to do one day but not really important to us now. I use many other forums and haven't found your suggestion adding or detracting in my experience. It's 'invisible' to my perception let alone useful. There are many things, simple to implement, that we don't go for.
We don't advertise PPRuNe, we don't issue press releases, don't allow any audio in ads and generally cut out every aspect of eye candy. No sigs, no attachments. Content and word of mouth has led to the site you see today - other sites see it differently and go for a far more intense graphic style and good on 'em. We've found that if we're running successful forums there isn't the time even consider the pretty stuff.
Many will have entirely different views but Danny and I didn't even bother to register on our own site until 5 or 6 years after it started. We plow our own furrow in life and I'm sure we seem pretty odd for not following the, graphics rich, herd. Can it change? Of course but only for content and not the periphery. Thus we're working towards enough server capacity to host video and pics within posts in 2008 without it turning the site to treacle.
Rob