Hi Personally I experienced a drop with one of my sites after getting a sitewide link. I submitted for a free listing in appropriate category but got a sitewide, due to some sort of new listing on every page. I saw a drop in SERP then, but I'm not sure this was the reason, because other site are trying too... Anyway if it doesn't hurt, the weight of the backlink may not be that much either... my 2 cents
It is good to have sitewide text links in niche directories. You will get more traffic from very niche unique visitors. There are 100 pages or 1000+ indexed in search engines, not only homepage. Nobody can guarantee that human visitors will find your website from only homepage, but more and more you will see your traffic from the deep pages. Good sitewide text link is good if it is not more expensive.
Yeah, depends if you're doing it for traffic or link juice. Like Directory4 said, you might gain some traffic from it. But as far as link juice, I think it looks more natural with just one link - especially these days when Google are on the hunt for link buyers/sellers.