As a performer, these days I tend to prefer gigs with guarantees, although as a some-time house concert host, I haven't been in a position to give them - and on at least one occasion felt pretty bad about that.
I wouldn't say it's exploitation not to offer a guarantee - everybody works in their own way - but these days I do tend to ask for one, and will tend to be more likely to pursue gigs where one is offered. If I am traveling significantly for a gig, a guarantee is necessary just so I can be assured of making back expenses. (All bets are off if I happen to be passing through your area anyway - so just ask me...)
Now, I have my own reasons for this. Since I have a more than full-time job, I don't tour all that often and am not likely to be as well known as some other performers. I now have enough credentials that audiences will often take a chance on me, but I still depend on a venue to do the bulk of publicizing the concert and to bring in an audience. A guarantee gives me just a little more assurance that they believe they can do that. (Of course, I provide resources, sample press-releases, and the like.) My favorite kind of audience is one who regularly attend a concert series no matter who the performer is, and are open to hearing a wide range of traditional and contemporary material.
I wonder if it would be helpful for your coffeehouse to run a benefit with local musicians, so you can feel comfortable with a little more seed money. If you have the funds to pay the costs, including guarantee, of the next two gigs even if they flop, you're doing okay.