Been running 0.11.1 since it was released for a while and only ever had blkxxxxx.dat in the "blocks" folder. Never had any revxxxxx.dat files until forced to re-index.
Some references:
http://bitcoin.stackexchange.com/questions/11104/what-is-the-database-forblocks/rev*.dat: these contain "undo" data. You can see blocks as 'patches' to the chain state (they consume some unspent outputs, and produce new ones), and see the undo data as reverse patches. They are necessary for rolling back the chainstate, which is necessary in case of reorganisations.
https://en.bitcoin.it/wiki/Data_directorylocks subdirectory
[v0.8 and above] Contains "undo" data.
rev*.dat
You can see blocks as 'patches' to the chain state (they consume some unspent outputs, and produce new ones), and see the undo data as reverse patches. They are necessary for rolling back the chainstate, which is necessary in case of reorganizations.
https://bitcoin.org/en/release/v0.11.0