24/7 is in fact a negative as the state of the DC Metro and NYC system show especially in comparison to Tokyo's multiple system.
There's not enough time to do proper repairs and improvements.
What I don't understand is why more cities don't operate like Philly and operate 24 hour bus service in place of the trains at night. With a system like that the night workers can still get to work as early as others and the bar crowd can still go out with out the craziness of last train like in Tokyo.
NYC has buses covering pretty much the same routes as subway, it just takes much longer. Personally I use express buses for commute and these are amazing, though twice as expensive.
Some metro systems have a 24/7 bus system, but Philly's is an exact replacement for the trains. They don't make all the extra stops of the buses that serve similar routes. I'm unaware of anywhere else that does this