Probably referring to Bay station being only a block away from the stations on either side of it.
Bay was built because it was planned as a terminus. Now that it isn't, it appears to be redundant.
But it gets a lot of use, and serves customers who would otherwise use Yonge, whose platforms are occasionally dangerously overcrowded.
So it's a good thing that Bay is there. It might not have been built if the planners had known it would be just another station on line 2. Consider it a good choice, by accident.
In any event, one extra station doesn't "limit its effectivenss as a rapdi transit system", even if you're thinking of line 2 an express route.
I've always looked at all the downtown stations as being close together to function like a local route, turning into an express route when they run beyond the city core.
If the stations were further apart, people going from/to downtown would have to do a lot more walking, and the stations would be ridiculously overcrowded.