Correct answer, but utterly useless references. The first doesn't even mention maintenance plans. The second says they can't be used to do bakups for a database that uses SQL Server Managed Backup to SQL Azure, not that they aren't supported at all (for example for doing database checks, or index rebuilds, or statistics update, anything but backups). So the references don't support the answer at all.
Perhaps a better reference would be http://msdn.microsoft.com/en-us/library/azure/ee336245.aspx#sqlagent which says that SQL Server Agent isn't supported, and since maintenance plans use (or used, last time I looked - it may have changed) SQL Agent for scheduling they won't work without it.