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Hi, It depends on the brand and the sector I think but as a rule it is not a bad thing to have control of your brand across domains. If financial considerations are not an issue I would keep them. 24 is not that many, I have seen a list with hundreds... with loads of different registrars and renewal dates.... it was a nightmare!

Sub domains are considered as unique domains in Google so at the moment your subdomain is not passing any link juice to your main domain. 301redirect from sub domain to subfolder might not pass all the link juice but still a better option for you as currently you are not getting any juice from your blog.