Why does Relic shield have a melee requirement for the execution of the minion?
First, HP is a pretty awesome stat on any support. Ranged supports tend to have the lowest base HP of any champion, which in turn makes flat %HP increases offer the largest %increase in defense on them compared to anyone else.
Really liked Relic Shield on Sona a lot more than the 2 alternatives.
Second, HP is really nifty and crazy cost-effective for the lanning phase. HP quadruples over the corse of a game from bases - where base damage only tends to triple and base AD only doubles. Most stats are priced at a ~level 9 zone, so good news for HP.
Third, and perhaps most importantly, Marksman.
Why run something like Doran's when you and your support can share a good chunk of the minion waves, with guaranteed last-hits and pretty impressive sustain?