What taping situations do you use them in?
The rule of thumb that I've always thought of is to use subcards in the same situations that I would use omnis
1. except, due to whatever reason, it is too difficult/impossible to split the omnis or too much hassle to run a Jecklin disc. There have been many situations where splitting omnis was impossible because it was difficult enough for me to block one stand much less two. I'm talking wide split > 8'.
2. or, like you say, the rear crowd/reverb rejections is desired.
These situations for subcards would most likely be outdoors, onstage, or indoors in a good sounding room up front.