It cheapens death when characters are resurrected too often, or purely out of sentiment. It's a trope that should only be used when there's a good reason, a good story to tell -- ideally one that doesn't just erase the original loss, because that's just a copout. Bringing back Cassidy as Black Siren worked because it didn't erase the characters' loss, but reinforced it through the contrast between the Laurel they wanted and the Laurel they got. Same with Sarge on Agents of SHIELD this past season.
You've got a good point here. Maybe killing her of was a good idea, but they did it at the wrong time. I would have done it later, with Sara staying a longer time in Arrow and joining the LoT in Season 2 or 3.