Its really a content balancing issue. You can't have any support class' group-wide buffs be too powerful for 8-man content or too weak for 4-man content.

For instance, if you send support_01 into Sashtasha HM and they can buff your DPS and Tank's output by 80% to make up for missing the other DPS, that ability would be ridiculously overpowered in 8 man content, buffing 3-4 other DPS and 1-2 other tanks! It would have to be time-restricted buffs to single players with weak non-tangible group buffs.