Arterial lines are not completely benign. They can definitely cause catheter-related bloodstream infections. This study shows that arterial lines have a rate of bloodstream infection of about 1 in 1,000 catheter days. Femoral arterial lines have a higher rate of infection compared with radial arterial lines (OR 1.93):
http://www.ncbi.nlm.nih.gov/pubmed/24413576