If you waste a weekend on something (fun), that's not really a significant loss. I'd be more than willing to waste a weekend building something even it doesn't work out. However, most of my partnerships are have lasted much much longer than that. It's very costly work on something for even a few weeks and have the partnership fall apart.

Just recently a friend of mine had a serious break-down with his designer. This wasn't a partnership relationship, he was contracting the designer. But when the designer flaked out, misappropriated other designs, and disappeared for weeks that ended up costing my friend a lot of time and money.