Keeping Acquired Old Domains?

[jmilgtp]

I own a dental agency where we buy out other dentists offices and operate them within our company.  We change all the branding and signs to our company name.  However, a lot of these dentist offices that we acquire have domains and websites that they've owned for years.  Some of these, despite no effort around SEO, rank decently well for local searches.

Currently, i've been keeping these old domains, changing the company name, contact info etc. and then backlink to our main site using various keyword phrases. 

Should I continue to do this, or would it be better to focus on our main site and keep building out location specific pages in the hopes that they will rank for local SEO searches?  If so, what do I do with the old domains?

Thanks!

[smartscraper]

if any website is ranking well, make sure you redirect the domain properly to get the benefit.