At the end of the day it's still work, and often a good hourly wage for the dancer involved. That said, if you tell them you've got a smaller budget, you can often get their models for a fraction of the cost you were originally quoted.
Agencies usually try and charge a premium cost for the use of their models and dancers, especially if they know you've got a big budget or you're a big name artists. As they know the agency however and trust them to make the right decision for them, this is a lot less of an issue. This will mean a lot of people not responding, and a lot of communications falling through regardless of how serious you are. Sometimes if you approach a model personally, they won't take you seriously.
While some agencies can still take a while to finish up booking a model, compared to the alternative methods below, it's still the easiest way to book your extras as the agency does most of the hard work. This is often a strong motivator, and will mean less trouble with regards to chasing up where people are. Agencies will usually have briefed their dancers and models that they need to be reliable otherwise they won't get selected for future work. Often going through a company will be the easiest route in terms of making sure everyone turns up when they're supposed to. Here are some of the pros and cons of using such a agency to get models for your music video: Pros: