I will preface this by saying that I do not like rangers, and I prefer not to include them in my builds.
Despite this, they are the only scout unit eldar has, which would be fine if they weren't so darn expensive. Scouts are dirt cheap, come with stealth, and the incredibly useful stun grenade.
Getting rangers can set my preferred build back because of their cost, and they don't really provide any utility except for their vision
Any chance Eldar can get another scout unit
Make rangers much cheaper, but make their guns an upgrade or something