It is a common practice to reface old cabinets when remodeling a kitchen. This can be a very cost-effective solution which yields an updated look. However, there are reasons to replace your cabinets. Spec cabinets are often put in homes for cost reasons, and may not be of the best construction. In addition, new cabinets can offer enhancements. New cabinets can be extended to the ceiling giving the homeowner more storage space. The cabinets themselves can be re-configured to more modern needs. For instance, new lazy susans are much larger those from 20 years ago.
It is important to remember that you will probably keep your new kitchen for 20 years. That is 20 years of daily use. Sometimes it is better to spend a little bit more upfront to gain a lot of practical enjoyment later.