There is two parts of answers to your question, but i can just summarise my points now:
1. Nekro shield= survivability
2. Nekro is cheaper than other pets because most people already have them

Nekro really revolutionised the gameplay in arcane legends. Be it pvp or pve.

In the past we make use of different pet AA to complete a map, pet changing is very common. But comes nekro (After all the changes into what it is now) which gives the strongest shield in game and stuns most of the time, it makes things a lot easier for the team, you can just use nekro throughout the map without changing pets for specific situations.

For PvP, the shield timing is pretty much what keeps the team alive in clash, without it, rogues just die seconds into the clash, no other pets are more necessary than nekro in pvp