Be it web application development in specific, or software engineering in general, or even in life itself, I'd say its much better to be good at many things than great at one thing. For one, it makes a person self sufficient and independent. Assuming one is a curious autodidact, one can always improve or learn things that they aren't good at yet. Next, it sort of gives you a perspective from various points that makes you appreciate things better. Finally, it is a humbling experience (which is good in life) as it makes you realize how much there is to learn - the more you know things the more you know you know nothing effect. Of course, one must not get perturbed when derided as "jack of all master of none". On the flip side, one must also not go around town calling oneself a polymath or a renaissance man - those days are over, at least with established fields of knowledge - as it would be equally ridiculous.