Adding art to your game really adds a pop and a sense that what you are making is something people want to play and see for themselves. Something you can show off at an expo. It is not always the best idea, however, to go and add art into the game as soon as possible.
Prototyping is where you first find what is known as ‘game feel.’ There’s almost no way to describe it other than that. It is the impression of how everything works in your game; the relationship between the player and the game itself.
You are going to need to fine tune this game feel as much as humanly possible. Even with play-testers or strangers at conventions. If there is a lot of flashy art and sound effects, they may become distracting. Please note, great art and audio will mold well with your game. Right now, however, we are specifically talking about prototyping.
If you really want to know if your gameplay works, show people your game with minimal art of squares and circles. If they find it fun and addicting regardless of the bland visuals, you’ll know you are doing something right.
Another advantage to prototyping without art is speed. You are a designer/developer. Maybe you have an artist who is willing to collaborate for free, or a partnership, and this fantastic. A lot of people don’t. So you can get people behind your idea without having to worry about art just yet.
Even if you are part of a team, you could always prototype and idea to show the rest of the team. It will be faster and easier without worrying about anything other than the gameplay. Then you could show your team and see what they think.
Artists, musicians, and sound effects artist should be praised for everything that they do constantly. Just like them, developers should feel like they can create and show what they have made without the others as a crutch. Go out there and make something great. The rest will follow.