Ranger is BY FAR the most powerful damage dealer in PT outclassing thieve and darknight. Only thing that comes close to matching the damage a good ranger can deal out is Black Mage's Ancient magic (flare, quake, etc) - but even that doesn't come really close, not to mention you cannot use such magic that frequently due to MP restriction.
People who don't know the power of ranger is simply a noob, in my opinion
However, a relatively new player SHOULD NOT play ranger. As some already mentioned, if played poorly with poor equipment, ranger will harm the entire PT by missing too much.
To be a good ranger, you need TONS OF GIL to buy good bow, arrows, and high AGI equipment. (Leaping Lizzy Boots is a MUST at lower level.)
My dream PT has always been:
PLD, THF, RNG (PLD for tank, THF-RNG for 2 Renkei (Sidewinder->Viper or Dancing)
BRD, WHM, RDM