Yeah, I think there are a couple of schools of thought on this. If you make it a feature and do it tastefully and in a way that makes sense I think you can get away with one every few months. On the other hand, one might look at it as better to keep your list focused on and engaged with your own music.

I think JV swaps are a great way to build your list in the beginning, but once you have a handle on your funnel I think PPC is just simply better and easier and there are no potential holes in your net either.

One really under utilized JV strategy is to build a network of 5 - 10 artists that you plan to perform with and team up to create a bit of a trade union. You each promote each other on a regular schedule and the eventual list crossover actually helps because when one member of this group releases something, suddenly people are hit by several emails all at once, making the release seem like a really big deal. It also really helps with increasing the pull you have to live shows because your fans get to see multiple artists they are familiar with, and you don't have to worry that you are losing leads to another artist because you know the promotion is going to come back. 10 artists with a list of 10,000 each suddenly becomes a network with a reach of 100,000 for the same amount of work and/or ad spend. heck, you'd even have the makings of a small touting festival on your hands if the music was niched out.