As long as the stock is moving well, yeah its fine but when it starts to slow and that intraday range gets tighter, you may want to move on.
One of the more successful traders I've heard of trades nothing but WMT. Although my understanding is he combines trading the options along with trading the stock so I guess you could say he's trading one company.
If you're talking about daytrading one stock, you're better off being more versatile. There are certain scenarios which a trader can miss out on if he/she chooses to limit themselves to a single stock. IE. triple witching, economic #s, news(!)...