If you haven't done it, you've been a victim of it. A well-meaning acquaintance calls and says they're looking for work, and thought to maybe sit down and have a coffee.
As a recruiter, I get this a lot, at least until I started making it clear how much I detested the practice.
The major problem with get a coffee, grab a lunch is the object of your affection doesn't need you to buy them a coffee or a lunch, and if they take it, they're taking advantage of someone who may need that money (someone looking for a job). Worse yet, a coffee is a major inconvenience in time, adding in travel time and mental focus, that at best, yields a good feeling for helping someone out with feel good advice.
Except, they don't want advice. They want a job. If you told them you don't want to meet with them, but you'll take their resume to a hiring manager in need, they'd be a lot happier. The coffee is just a way to make it feel good, kinda like taking a Craigslist hooker out to a nice meal before heading to a hotel room. You may fool yourself into thinking it's a date, but that's just social niceties.
And those are the ones you can empathize with. The worst are the ones who are just looking to appear busy. I say the worst because I've been that guy. I scheduled a meeting with a guy, to grab a coffee, way back in 2002. He was a small business owner, and I was a staffing salesperson. After an hour of me talking about ideas, he stops me, looks right in my eyes, and asks me what he was doing there.
I was wasting his time. I was using him as a sounding board without any benefit to him. He politely waited, and then called me out on it.
Since then, I've tried to never do that to someone else, but I've had it done to me dozens of times. I can't complain, but I do instruct. And that instruction usually is to get to the point, and to do a lot of prep work if you're going to ask someone to meet you.
When you're looking for work, you need to be respectful of the time you're asking of those who are employed, not because they're more important, but because you want something from them. What you really want, is them to break out of their comfort zone, and actively work with you to get you work. You want to impress them, help them, and persuade them that it's worth their time to do more than mouth platitudes or forward a resume.
Showing up at a coffee isn't enough. Offering to "help" them or "network" with them isn't enough. And God help you if you haven't prepared for resume, practiced your elevator speech, or don't bother to show up early.
1) Don't Ask For A Meeting Without Telling Them Why. Tell them you're looking for work, you recognize it's always a drag to have to meet someone like this, but you've got a series of questions that you hope will make the time interesting, and worth their while. Set the time in a way that is convenient to them. Stop talking. Listen to them. Don't cut them off. You're asking for them to help you. Don't make that painful.
2) Make the meeting about them. Ask them how they got their current job, what they think of their industry. Ask them about promotion, hiring, the market, and what their biggest problems are. You do these things to learn, to get better at learning, and to take the knowledge you're given and apply it to interviews. This information makes you useful to the person you're meeting (giving them introspection), and it makes you interesting in your next interview.
3) Keep it short. Tell them 30 minutes, and at 20 minutes, remind them the 30 minutes is almost up, and you want to be respectful of their time.
4) Be prepared. At some point, they're going to ask you what they can do for you. Be as specific as possible, down to giving then names and companies you'd like them to make introductions for you. They can say yes or no, but if you ask them, you take away the vagueness, and show how committed you are. You're proving to them that if they do recommend you, you'll make them look good. Have your resume ready, but don't force it on them. Have a meeting agenda listing what you'd like to accomplish. That is what being prepared looks like, and it gives confidence to those wanting to help you.
Don't ask them to help you. Tell them what you'd like them to do to help you, and make sure they understand "no" and "I don't know" are acceptable answers. Your golden moment is to get them to pick up a phone and call someone to refer you. Not an email. Not a vague promise. A specific call to tell someone to interview you.
That's how you make a coffee worth their while.