The cost of owning a domain name and having a up-to-date, functioning website is a mere pitance compared to what you are losing from not having one.
If you are piggy-backing off another domain, with a sub-standard website, or even worse than that no website at all. Your customers are likely to have little or no respect for your business and, even worse, be put off entirely.
The reason for this is simple. You havent invested in your own business, so why should they. This shows a lack of professionalism and is an indicator to many of untrustworthiness.