I see no harm in one having basic experience in multiple fields. Especially when those fields are related. Actually, this concept I kind of "market" it among my circles. A mobile developer should know the basics of web development and vice versa. That way they communicate better.

I claim that already happens naturally. It's our drive to quickly build a niche (e.g. "I'm a professional X developer"), in addition to insecurities that lead to saying "X is not my responsibility", is what gets in the way of expanding our fields of expertise.