It would be much harder to get witnesses to flip against him.
Another problem is that it smacks a bit of "Show me the man and I'll show you the crime."
Finally, making sure that an investigation doesn't appear partisan is not a bad thing.
"Appearing partisan" undermines the entire investigation in the public mind.
The DOJ (and prosecutors) has enormous power.
Using that power cautiously isn't a bad thing.
On the other hand, being like Trump is bad.
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