Guest blogging is not a waste of time, however, the caliber and credibility of the site hosting your guest blog is key to the value. Run a backlink profile of the site that you wish to submit a guest blog. If they have spammy backlinks, do you really want to work with them? Look at the other guest articles. Are these of the same quality as yours? Would you want them on your site?
If the guest blog host is reputable and they can directly benefit from the expertise you provide and also naturally has an affinity to your site, then this is worthwhile.
