Hello David
The aggregate-address command is used to create an aggregate entry within the BGP table. This essentially means that you can summarize multiple routes into a single route, thus reducing the size of the BGP table and minimizing the number of BGP updates when individual routes change.
If you issue the aggregate-address 0.0.0.0 0.0.0.0 command, you’re telling BGP to consolidate ALL ROUTES into a single aggregate route. This is essentially a default route FOR EVERYTHING, and in the context of aggregation that doesn’t make sense. Taking aggregation to such extremes defeats the purpose of the feature.
If you want to create a default route, it is preferable to use a network statement in the BGP router configuration mode. This is much more efficient (aggregation requires more system resources to perform) and simpler as well.
Note that the use of the summary-only keyword makes no difference to the outcome of the command. You get the error message with or without it.
To be honest the error message is a little bit out of the ordinary, I’ve never come across an error message that says what you’re trying to do “makes no sense”! 
For more info about this command, take a look at the following command reference:
I hope this has been helpful!
Laz