For a newbie, who is not whaling hard, you will want to get the most bang for your buck. So, you'll go into the 'use gems for summoning heroes' mentality.
However, for those who have already crafted their PvP teams, the main staying point is to win PvP battles.
And since you can't really farm for 5* equipment, and materials to make a single piece of 5* equipment is difficult to farm as well, its ok to summon them. However, due to the recent patch that allows equipment to be upgraded to +15 and then gain a star improvement, makes it seem similar to the hero side... except equipment can't go out to fight by themselves
Added to that, the effect of sets...