It's easier to focus heal one target than multiple, so as a healer main I appreciate the party letting the tank pull exclusively. Personally I love big pulls, especially at the end of an patch/expansion when ilevels allows speed running with minimal GCD healing. However, if the tank wants to go one pack at a time I'm not going to pull for them encourage the dps to pull. If dps is decent then the difference between large and small pulls is at most 5 minutes (at the most extreme. It's usually 2-3 minutes I find) but a wipe can cause a much larger setback.