Its really up to you to accept them or not, 95% of them will be garbage as you correctly said they are all to do with building back links for seo.
Its not 'black hat' seo as nothing sly or shady is happening and its certainly not in violation of Google guidelines* (read below for more info on that).
An awful lot of big national news sites and portals publish guest posts / guest content, and they can be really interesting, but do pre vet the articles as for the most part they are just churned out junk, as a vetting process i would recommend looking into the background of the person asking you to publish there post, if they are from the same industry / niche as your site and the post is interesting, your audience may find the article interesting, on the other hand if the article is from an seo company they usually wont be up to scratch, or relevant, although larger agencys repressing larger companys may still have interesting content.
*What you have to look out for is when they offer money in exchange for the article being published - this in it self is not a problem but if you / they include links in the article and those links pass 'link juice' ie they are not
rel="nofollow" this is explicitly banned by Google's guidelines - for more info on that see this article on search engine watch.