No, that's the point actually of why Redirect is a good thing.  There is more to it than just relevancy and similarity.  As I described above the PR of the site you're linking to has a big impact as well.  So they can be relevant/similar, but if they aren't discriminate in who/what they link to you can actually hurt yourself much more than any potential PR gain.

About the spider not able to follow a link to that relevant page...not sure I understand exactly what you're asking....but making a guess at it:  That's the same as what happens with a nofollow tag -- nothing.  It doesn't affect your PR because you aren't bleeding PR.  The thing to keep in mind is that you -- if you care about your PR (meaning that you are concerned about it, worry about it, want to do things to improve it sort of caring) -- should try as much as possible to control any PR bleeding and should carefully control what outbound links that you do have AND as much as possible control who links to you.  One thing that I think that many people just don't get is that you can actually have a higher PR with relatively small number of links...just have few but excellent links. 

I think that I posted this here somewhere but if you were really wanting to maintain maximum control you could achieve an extremely high PR and have no outside links AT ALL...other than to your own sites which are relevant/similar (e.g., create say, 25 similar sites all with relevant (but at least some content differences).  And improving upon even that would be to have each site ONLY outbound to one external but highly ranked site.  This is counterintuitive but you can actually make greater gains that way -- and I have for one of my own sites targeted a "prime" external site to link to, who wouldn't agree to links to/from people and I got one after sending emails back and forth about my desire to ONLY link with their site.  They gave me the link back.  Big jump in stats followed.

Actual mileage may vary...but that's the gist of it.