Let's be honest... In todays world, a website is a reflection of your brand. A good website yields a positive reaction. The viewers buy more, stay longer, and feel better in general about what you are offering.
On the other hand, an ugly website that offers a bad experience coupled with slow loading times always conveys a negative message. Don't let your customers self qualify and leave. In general, a bad experience gives the impression that you simply do not care about them.
If you answered NO to any of the questions above, it's probably time to consider a redesign.