I think it's good to do the Necromancer task at least once due to you have an opportunity to kill Necropharus. The best thing he drops is the Soul Stone. You can only kill the boss once but you can take this task again after you complete it. Doesn't hurt to do everything once. You can also take your time in doing this task, you don't have to go overboard and just kill Necromancers the whole time. It took me years to finish this task once because I didn't focus on it. You can take it and kill them as you go!
The Soul Stone is indeed cheap on many servers thanks to the Percht Queen and among other bosses dropping the Soul Stone.
The other reason why it's good if for the experience, sure the Soul Stone is cheap but back in the day if you completed this task you were a Tibian Hero. Everyone was shocked if you actually killed that amount of Necromancers. They eventually upgraded the Necromancer task to include newer creatures in 2013 like Blood Priests, Blood Hands, and Shadow Pupils. Thus making the task way easier.
It's great for bestiary! It motivates you to kill other creatures so more power to you if you want to gain some charm points.