I'd say the spirit of open source is that others are free to modify the code and that's it. This requires a good license, the possibility to fork, some documentation and a way to build the project yourself.
But why would accepting contributions be required?
A lot of people are very entitled. They think that an open-source project gives the right to make request/demand of the project. Even if they are willing to write the contribution themselves, they still think they have the right to have their pull request accepted. They forget that 1) it may be outside the scope of the project, and 2) the project owners are going to be the ones that have to actually maintain the code they commit. Crazy stuff.
I'd say the spirit of open source is that others are free to modify the code and that's it. This requires a good license, the possibility to fork, some documentation and a way to build the project yourself.
But why would accepting contributions be required?