A simple text based format is needed for displaying points easily (or WKT encoded geometries).
Three main use cases:
1) Novice user wants to create data from scratch or geocode some tabular data - its small and already in a spreadsheet, why not just allow them to read it and render it directly to avoid conversion step? This user could then push their data into google docs or even version in github so multiple users could collaborate on the simple file and rendering could get live updated. Mapnik could gracefully skip invalid rows (verbosely) and then errors could then be corrected at the source rather than just in the conversion step (to some database).
2) A lot of API's dump data as csv. These same API's should support json, but until Mapnik adds a fast, native geojson plugin, a fast native csv plugin can suffice for optimized rendering of small data chunks (by catching in memory at first load).
3) Massive government data already in csv with lon/lat. User wants to be able to look at it before trying to make sense of it more, and it is so big that normal spreadsheet or conversion tools fall down. We can efficiently render the bits of it that seem valid to enable better data exploration.
And specifically re GeoJSON . It is great, but:
So, a native CSV (e.g. tabular plugin):
work underway on this at https://github.com/springmeyer/mapnik/tree/csv_plugin
now as a branch here and pull request is queued up: #912 for post 2.0.1 release merge.
merged into master in c97c4c9, closing!