Dispensers for monomer are not really recommended. Liquid always seeks the lowest point, if it comes out it will go back in (gravity is an amazing thing). YOu run more risk of contamination and service breakdown when using a dispenser, and could (though I have not heard of it happening) cause cross contamination from client to client. Monomer won't support bacterial growth so it wouldn't be in the form of an infection, but you could get oil, dirt and other particles in your monomer and cause some service breakdowns.
A dappen dish with only enough monomer for one client for that service is recommended for infection control and client safety. You also usually use less when you put the recommended amount into a dish vs. the dispenser. It will evaporate from the dispenser. Some people really like their dipensers and they work well for them. I found it was too expensive to replace contaminated monomer and that I went through monomer twice as fast. I also have better control with the dish, I can pick up what I need, make sure that my bellly is full and I end up with the perfect mix every time (if I pay attention to what I'm doing that is)