From the cop's point of view:
-It's an easy arrest.
-It shows proactive policing, which is pleasing to the brass.
-It's used positively in the precinct's weekly CompStat results, which is pleasing to the brass.
-If arrest is made close to end of shift, you're talking minimum two hours of OT for paperwork and processing.
-If charges are dropped, it will be weeks, months from now - an eternity in cop time - and no one in the brass pays attention anyway.