>This idea of "adding" sentences is ridiculous, and my guess is it's only (ab)used as a way to force people into agreeing to declare themselves guilty, and the prosecutors going "easy on them" and only asking for 30 years in prison, instead of 100, if they win.
Unrelated to this case, if you don't do that, won't it encourage people who are knowingly committing big crimes to commit smaller crimes on the way? Why would you want to essentially grant immunity because someone committed a bigger crime? Where's the deterrent?
Your question boils down to, "Is a 105 year sentence a better deterrent than a 25 year sentence?"
To answer that question, I'd direct you to a 2010 paper from The Sentencing Project. The thesis presented is that the certainty of being caught is a greater deterrent than the severity of the punishment. Further, increased severity does not appear to have a significant effect on deterrence.
Unrelated to this case, if you don't do that, won't it encourage people who are knowingly committing big crimes to commit smaller crimes on the way? Why would you want to essentially grant immunity because someone committed a bigger crime? Where's the deterrent?