Fr. Karban makes many good points about equating the stipends with sale of indulgences although I see this as a false equivalence. Many of us see the stipends as simply a donation to the church through the priest who will say the intended Mass. I am happy to provide a donation to a priest who, although salaried, likely could use a few extra dollars and many I'm certain just donate the funds to the parish. Although technically the Eucharist is there for us all regardless of donations, the fact of giving something of worth, as in "time, talent, and treasure" to the 
church enhances our feeling of belonging to the local institution.