It's an important illustration of why context is so important and why decontextualized readings can be so misleading. Often this verse is trotted out in discussions about reimagining church. "You know, two or three gathered in a coffee shop could be church!"
Really?! I am all for reimagining church, but let's take scriptural guidance a little more seriously please. Let's look at the context, let's look at the full dialogue. Here's the full text of Matt 18:15-20, this time from the NIV (for ease of reading):
"If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over. But if he will not listen, take one or two others along, so that 'every matter may be established by the testimony of two or three witnesses.' If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat him as you would a pagan or a tax collector.
"I tell you the truth, whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.
"Again, I tell you that if two of you on earth agree about anything you ask for, it will be done for you by my Father in heaven. For where two or three come together in my name, there am I with them."
When the full text is read it becomes clear that the context is one of church conflict. It's not about missional worship gatherings it's about conflict resolution gatherings. It outlines a staged process of escalation. First, don't gossip or back stab but go directly to the person you have a disagreement with. Second, if you can't resolve the disagreement between you, get some elders to mediate. Thirdly, if all else fails, the community as a whole decides. If the infraction is serious enough and if there is no resolution, it's time to admit the community bond is broken. But lest this sound too harsh, again think context. Take note of how Jesus treated pagans and tax collectors.
So the final verse, what's that all about? It's an affirmation that where you can resolve conflicts at the early stage, without embroiling the whole church, God is with that. By implication, the two or three gathered are not the whole church. So however much we'd like it Matt 18:20 cannot function as a church formation procedure. Wherever we get our church formation theology from, here not be the place.