ESRI's documentation articulates it best:
"Imagine that there is a regular supply of text files containing geographic locations, but the data in the files has an unusual format. This data should be used in ArcGIS, but converting data each time a new file is received is impractical. In short, ArcGIS should work directly with this data, as it does with other supported data formats. This can be done by implementing a plug-in data source." (Source: ArcObjects 10 .NET SDK Help )
It would be nice to be able to work with custom data formats in the new ArcGIS Pro without the extra steps of import and export. Could something like this be implemented as an add in or extension to the Pro version?