1. Primetime's STA engine is a lot faster
2. More accurate delay calculator since the tool is aimed at sign-off rather than timing driven optimization
3. Support for signal integrity problem analysis (if buy the primetime SI license)
2. Although pt_shell and dc_shell apear to support the same commands at
a first glance primetime actually has many features and commands for timing
analysis that DC does not. For example, on chip variation analysis, interface-logic-models, several other types of timing models, advanced reporting capabilities.
There are probably many more reasons...