We are getting requests for apps which haven't been updated since Splunk went from 5.x to 6.x. Besides the fact the app may not run (not an issue with this particular app), is there any reason to believe that such an app would impact our splunk infrastructure because it is a 5.x app? I know some apps will cause performance issues because they have poorly designed searches or didn't use such things like summary indexes. I would argue that even Splunk 6 apps could cause similar issues.
The main thing is that the app might not run correctly because Splunk has moved some stuff around. For examples some logs have changed format and location (they added _introspection), etc. Personally, I would have no concerns about an older app causing a problem with your Splunk cluster, and have never seen one do so, but this is a big judgment call so i would get some other opinions.