Yes off course it devalues the passed link juice by adding parameters. But it is always a good idea to add parameters. As many times I had seen the cases that Google sees the affiliate links as a paid links which results in the big drop in rankings. That the reason the idea of using link parameters for affiliate program works well.
Google sees affiliate links as a paid links and you know what it means.Thanks to link parameters or affiliate URL's which saves our main URL's to get participate into bad neighborhood.
And its always good to save website's image while leaving link juice.
Mattcutts once stated Typically, we want to handle those sorts of links appropriately. A lot of the time, that means that the link is essentially driving people for money, so we usually would not count those as an endorsement. in an interview with Eric Enge.
And in 2008 in SMX Google and Yahoo agreed that they will pass a link juice from good affiliate networks.
Read this point from Rand's post.
Affiliate Links
Shockingly, when asked point blank if affiliate programs that employed juice-passing links (those not using nofollow) were against guidelines or if they would be discounted, the engineers all agreed with the position taken by Sean Suchter of Yahoo!. He said, in no uncertain terms, that if affiliate links came from valuable, relevant, trust-worthy sources - bloggers endorsing a product, affiliates of high quality, etc. - they would be counted in link algorithms. Aaron from Google and Nathan from Microsoft both agreed that good affiliate links would be counted by their engines and that it was not necessary to mark these with a nofollow or other method of blocking link value.
But note the point they had not mentioned what will they do with low quality links.
From the above points it clear that Google will passes a link juice. But still many of us in affiliate industry uses a parameters and redirects in affiliate urls. Reason is just simple not all the affiliate are as genuine or reputed as Amazon. So if your links in 50 sites and may be 40 site can be those which Google does not like so links from those site may harm your site.
So as I said above its always good to save website's image while leaving some link juice.
