(computer can do more precise calculus in less time, so why bother?)
There's always the KISS philosophy. Keep your systems only as complex as they need to be to accomplish your goals. If your goals are a simple combat system, then this would be an excellent option. If your goals are a realistic, intricate combat system, then something more complex is necessary.
For any combat system, there are two fundamental stages that are necessary. Determining hit or miss, and determining damage done. Beyond that, you can add any number of additional features to reach the level of complexity you wish to achieve.