Do vultures suck? Yes. Are they doing anything illegal by standing around in a point of their choosing, doing absolutely NOTHING to hinder the fight or interfere with the combatants? No. Never. Not in a million years. At best it's rude, but so is /check according to some people =/.
Get enough people standing around and lag starts happening. Historically, some have issues with avatars causing FPS chug, especially with Titan doing something like Earthen Ward to everyone it can. When people start lagging, their performance starts slipping, and given FFXI's age and design, getting a better computer isn't exactly a guaranteed solution, nevermind the asanine implication that someone could have to spend hundreds of dollars on a new rig to help keep claim or guarantee the success of your party/alliance I'm sure would be suggested in counter. Plus if it's a location where adds are an issue, you make it more difficult for the main fighting party to tab around quickly. So no, they're not being harmless.