When I am growing a multi-resistant bug and I am symptomatic (increased cough and sputum, change in color and consistency of sputum, shortness of breath, wheezing, decreased PFTs and O2, chest Xray evidence).
If several bugs of varying resistances are growing at the same time (one strain is sensitive to Cipro and Tobi), we try that route first. Regardless, if I am getting sicker and sicker and nothing else helps, I go on IVs.
This past year and a half, I have been on IVs 3 times (only go into the hospital to get PICC, get a few days of HEPARIN and watch for clots - I have had two clots), which is the most frequent it has ever been for me. It's because I have a strain of PA that only responds to Ceftax/Tobra/Gent. And it scares the hell out of me now.