I'm the first to admit, the actors not recording together, gave the animated series a very stilted sounding dialogue track, and it hurt the show.
While having actors record their lines at different times (or even at different studios in different cities) is fairly common in V/O work, not everyone does it well; it's like working with a green screen, only you're standing in a cubicle with a script and a microphone and everything else -- other characters, sets, props, everything -- is inside your head. Some with good imagination work well this way, but most voice actors benefit from working live with the other actors, in the same way that live radio theater was done. Wil Wheaton had a good column
recently in which he talked about how different acting is without the physical and visual aspect, and how hard it can be to convey those using only the voice.