There are times where it is convenient to define a plugin for all child projects except for a few. Adding a skip attribute at the plugin definition level (similar to inherited) would allow for the addition/configuration of a plugin in a parent pom that would be inherited to all children. For the few children where that plugin either doesn't make sense or causes needless executions or errors, this would allow for it to be skipped.
There are a few plugins that already include skip as a configuration option. Adding it at the higher level, making it available to all plugins would allow any plugin to make use of it and not have to define it's own skip configuration parameter.