Firstly, losing
PR value as a possible result of OBLs on your homepage doesn't necessarily translate to lower SERPs, I think it's universally agreed now that
PR shows less relevance to the algo rating of your site.
I've seen a site with dozens of OBLs on homepage, steadily holding its PR6 despite everyone else devalued in the recent update. I've seen other sites participating in a nasty link exchange network of flashing buttons neatly arranged at the foot of their homepage, still ranking well despite the obvious link-farm network.
Personally I avoid this but do include some obls on my homepage, like sponsored ads (nothing wrong with that) but avoid whole lists of obvious link exchanges.
The one certain thing to consider is that obls on a page drain the
PR that percolates down through your site, what ever
PR is on the page has to be shared by all links on a page, therefore the more there are the lower the
PR within your site. If you are a PR5 with a sensible number of links then the next level pages will all be 4, if you've SEOd it to death and given away a lot of your
PR to OBLs then expect deep pages to be PR3 or even 2.
I also agree that Google respects a site that is leading the reader to other resources on the net and for this reason will always add one or two arb links to external sites at the bottom of content - without getting links in return from them.