The good side is that it offers a chat function outside of federations, which are more of a military alliance. It also appears to somewhat increase the profitability of corps that sell their goods in one and, with advanced understanding of quality, can be a way to purchase products you need.
The bad is that without an advanced understanding of what quality you actually need things at, you can do some real damage to your economy. Some considerations for purchasing items for your government can be found here:
https://www.simcountry.com/discus/messages/16295/25631.html?1459724885 For supplies to corporations, achieving an average supply quality (weighted by unit cost) of close to 215 is the most profitable. Supplying your corporations through a common market often results in purchasing supplies at a much higher quality causing the corporation