While external sources is a great feature, it can generate a lot of query terms when a client is configuring many external sources. This slows down the queries and can have a big impact on performance.
When a client's external sources are all in a single collection or in a few collections, it would be simpler to him to specify a few external collections instead of a lot of external sources. This would generate less query terms and improve the performance in the same time.
|I need it...||Not sure -- just thought it was cool|
Makes perfect sense, we'll include this in December release.