It used to be a prerequisite of being an ECB premier league that you had a 2nd XI's set up. Playing Yorkshire prem I found the standard of the 2nd XI league deteoriate due to the problem mentioned until we found that the 'power' clubs (Sheffield Collegiate, York etc..) were pardoned from playing in it. Standards got worse and worse until the ECB agreed that Yorkshire could disband the league and only have a 1st XI league.
Whether the ECB have changed the rules, or whether some county boards are just allowed to do as they please, I'm not sure.
The Northern League currently persevere with a 2nd XI league, and again the standards vary wildly between teams.
Personally I think allowing your 2nd string to find their own level in more local competitions is a better approach.