Great points all Miriam!
Many local businesses and SEOs have a Google strategy of divide and conquer.
With Google Local a better strategy is "United We Stand!"
The other important thing is that we need to be mindful of the S in NAPS (Name, address, phone, site) All need to tie together.
One well optimized site with location pages works better for a wide variety of reasons.
Here is a biggie that many don't realize. Let's say main site is optimized for all the plumbing KWs and Place page is linked to home page of main site.
Then you do a microsite for citywaterheaterrepairs.com. If NAP is on the site and other signals to help it rank locally, that tie it back to the main business, sometimes the algo will REPLACE THE URL on the main Place page with the microsite URL. What happens next? The ranking drops for the money KWs- city plumbers, city plumbing service etc, because the Place page is now linked to a site optimized for waterheaters.
I've had SEOs post in my forum about this problem and even post emails they have gotten from Google support saying they will not fix the URL. Support basically tells them the algo will connect whatever site it thinks is relevant and if you only want the algo to connect to your main site - then you should only have one main site.
So anyway that's just one potential issue that can come up. Hope that helps and best of luck!