Singing wise- You need a vocally strong Ben and Sally (If you have "eh" singers doing "Too Many Mornings" then, well, it would kill it) Buddy doesn't have to be the best singer, neither does Phyllis. Heck, "Lucy and Jessie" consists of like, three notes. BUT they must all be good actors as well. Because if they can't act then watching the show would be just plain boring. It's two and a half hours of bickering. If the actors aren't good while doing it, then what's the point?
The same could be said with the score. Yes, you have to act the songs, but certain songs call for strong, or somewhat strong, singers. "I'm Still Here", "Losing My Mind","Too Many Mornings" etc.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.