Probably mostly positive actually. Removing people who either have poor self control or little ability to find productive work means they don't teach those habits to their children and those around them.
This does not mean the answer is executions though, as people's lives and happiness have value even if they're scumbags.
> What are the aggregate consequences of such policies to society?
Might ask yourself the same question about shoplifting gangs being let loose
And if Riker's is a problem, then deal with that specific problem. Not prosecuting people seems to be one level removed from it. (yes, they are connected, but for example, not sending pre-trial people there could be a start)
This does not mean the answer is executions though, as people's lives and happiness have value even if they're scumbags.