I tend to decline playing a reserve team when I'm at a non-league club, costs money to run them, kills your youth players & I never have a wage budget large enough to have a squad that would require a reserve side.
Pretty sure not fielding a reserves team saves you money, which is handy if you're a lower league club with a small squad/not enough youth to justify it. Generally you can't say no to an u18s team as the board insists on it.
I'll almost always go with both a reserves and u18 team. The only reason I wouldn't is if I'm a lower league side trying to save money. I can't work out why a Premier League side would decline a reserve or u18 side. I find that both prove to be good investments if used correctly. A lot of people seem to have the opinion that players don't develop playing in either u18s or reserves, which isn't really true. Giving them first team football or loaning them out for first team football will help them develop at a faster rate, but if you look after your youth/reserve teams and keep morale high, they'll develop there too, with the added bonuses of being able to be tutored and also have their training under your guidance.
When I'm a lower league team, I don't enter the reserve league. Instead, I schedule lots of friendlies against reserve sides of the major clubs in that country (e.g. managing Southport, I'll schedule friendlies against Liverpool Reserves, Man Utd/City Reserves, etc.) and play what would be my reserves/U18's there. I don't really mind how they do in them, but the money it brings in is ridiculously good for the club.
I'm slowly getting in the habit of using my reserves as a "Senior U-18" team for my EPL teams, graduating my more mature 17-18 year olds there to make room for new, younger signings. This is especially true if I am still working on getting them homegrown status, if a player still needs tutoring or targeted training before going out on loan, or if I am competing in the NextGen series (U-19 Champions League). Senior team members make appearances as needed to maintain or rebuild match fitness. Given my tendency to have too many youths on the payroll, I can't really fathom not having a Reserves side.