Are 301 redirects good for SEO?

The short answer is no.

301 redirects leak link juice, in fact all links leak link juice but a 301 leaks twice, once for the original request that results in a 301 status code and once for the second request that results in a 200 status code assuming it is successful.

Is cutting your hand off a good thing?

Of course no sane person would want his hand cut off, but if your hand had festered and it was poisoning your blood, then you may decide it is the best thing to do. 301-redirects are a simular story, you don't want to use one unless the alternative is worse.

Un-Necessary Redirects

Un-necessary redirects are usually redirects from internal links to the correct internal page. Obviously if the link and the destination page are on the same website then you should link directly to the correct page. Some of these 301-redirects are automated on a mass scale though code, sites often have 1,000 or even 10,000 plus un-necessary redirects. I have seen sites that 301-redirect all URL’s to have a trailing slash, that's fine in itself, I recommend that you should redirect either to have all trailing slashes or all non trailing slash URL’s, but you should then use those URL’s in your linking you should not rely on the 301-redirect to redirect them to the trailing slash or visa-versa depending on your choice. WordPress and several other CMS sites seem to do this a lot, either though the CMS itself or from the use of plug ins. The same rule can be said for canonical domain names fixes using a 301 redirect, once you decide on a canonical domain name stick to it and use it in your internal linking, you should also use lowerecase URL's to avoid un-necessary redirects. 

What about rel-canonical?

Yes they leak also, rel-canonicals should also be used sparingly.