My opinion: recently_used_apps is, as Allen Liu mentioned, from hardware inventory. The reason (again, imo) to enable a Software Metering rule is because then, and only then, will over time the Software Metering reports, which are based on summarization routines, build up meaningful reports. If you were to look at the Software Metering reports, you may notice that without defined rules, they may have little to no data in those reports. Add some rules, and check back in 2-3 months, and you will see a difference.
It may be that you don't need the SW metering reports to have meaningful data; if not, then yes, (again, imo), there is no need for SW Metering rules, and if it's something like... "Boss asks you for a 'quick check, how many people launch widgets.exe'", then you can use the data already there in recentlyusedapps. If instead you are on a 6 month+ project for "... we need to know who uses super-expensive application widgets.exe, to see how cost effective it will be to replace widgets with the application FuBar", then a sw metering rule might be a better fit to gather better information over time.