by getting incoming links, you're adding PR to the page of your choice..
depending on your site structure, you could get a net gain of PR to your target page.
and also increasing the keyword weighting, by using relevant anchor text
plus - if you're trading with related sites - you're helping visitors who might otherwise just leave your site.
they might be impressed by your links resource, and link to it from their own websites.
i always build links pages to try and rank for the theme of the links page (ie, outdoor events links) which our site is related to.
so, some visitors might visit your links page having found it through a search engine - and decide to stick around and check your site out (as its related to the links page...).
(having outgoing links to some quality sites and authority sites might be a good thing even if they are one way links - i dont know much about this though, but would appreciate any input at all on this side of things - such as, what sites are quality enough to link to)
im sorry i cant answer your question. but this is the closest i can get to a positive reason to link trade.