These very same people would be contributing to growth if they worked in the private sector and produced goods and service. Instead, they create either zero or negative growth.
Clearly you're going to end up with lower economic growth.
The fact he would spend his salary on purchases is small comfort, especially when he'd likely earn a larger salary to spend in the private sector.
It's not clear that you're going to end up with lower economic growth. For example, taxing some level of income that would otherwise not be spent on economic activity (e.g. savings or equity investments) and spending it on things that do result in economic active results in higher economic growth.