Here's a simplified way of deciding whether your organisation should start a community.
You need 1 full-time community professional.
You need to give him/her 10 months to develop the community to critical mass from a standing start. S/he will need to build relationships, help develop a platform, and proactively build up activity.
And it's going to cost you around $100k to get there. That covers the community manager's salary and cost of getting a simple platform up and running.
If you're a passionate amateur, you might be able to do it in less time and for less money. You already have existing contacts to invite to join the community and your platform requirements will be far less.
If you're building a community for an organisation, this is a good benchmark to begin with. If you can't provide this, it's probably best not to develop the community.