In short, route-cache keeps a traffic-drive cache of most used destinations, that is aged periodically and doesn't really take much memory. CEF instead builds a table for all the destinations using prefix-based algorithm (Patricia trees) that you can search about.

The reason you disable that, is because certain features (less and less, but still there are some) don't work unless you do that and fail silently.

Hope this helps, please rate post if it does!