Writing a script can be fast to write, simple and easy to create, and great for small tasks. Scripts are also very quick to test.

Plugins often require longer to develop, and can be a nuisance for a programmer to have to take the time out to write. A script can be written by a technically competant artist, or a technical artist (for the more complicated tasks), artists which are very close to the problem that requires solving. For tasks that require brute force and massive computation, a plugin will be faster at doing this task.

For the developer, plugins provide an important functionality, they are compiled, obfuscated so that any clever code you write is mostly secure from people stealing it. One of the issues with scripts is that when you distribute them, other people can take the parts of your code that took a while to figure out, and use it in their own scripts and not give due credit. Also as plugins are written in C++, the programmer works in an environment that they are comfortable with, and can use code and libraries they have previously written.

Perhaps the most important win for plugins is that you have a deeper access to the underlaying workings of Max. Many of the functions of the Max SDK are just not available in MaxScript or even have an equivalent.