Depends on the skill of the rangers player, and gear. To me a good tank can grab and hold agro. if no CC grab and hold aggro on multiple mobs. I played healer for more then 90% of my eq life. I would rather heal one guy with multipule mobs on him. Than 2 guys with one mob on them each.
I play a Bard, I've tanked almost the whole way to 50. I've yet to group with anyone that can hold agro off me when I want it. This includes a raid tank for a specific guild... he knows who he is.
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Honestly though it comes down the player skill, and how much you work on your gear. I've seen rangers out tank pallies, sk's, and warriors - Raid excluded. Although for raids they still have their uses as a minute man for rezing your actual tank/tanks if needed. Called minute man because they usually don't last much longer after.
So yes, for groups they are decent. Not the best, but decent.