Reasons why 'Default' is better than 'Main Build':

1. The latter uses the word 'build' which it isn't.
2. 'Default' is a better name than 'main'. This plan always starts as the default, but main doesn't really make sense for projects like Confluence. Every build in the project is as important as the others.
3. If we simply replace 'main' with 'default' and 'build' with 'plan', it has a redundant word ('plan') when shown with a heading or used in a sentence:

Web UI: "Plan: Default Plan"
IM message: "Build 1029 for Confluence project, Default Plan plan failed."

Both the above would be better without 'Plan' in the default name:

Web UI: "Plan: Default"
IM message: "Build 1029 for Confluence project, Default plan failed."

4. Encourages words that are simple adjectives for plan names. For example:

Web UI: "Plan: Functional Test"
IM message: "Build 1029 for Confluence project, Functional Test plan failed."