IV's every 3-4 to months regardless of health is called the Denmark way. This technique was pioneered there, and has shown to prolong life of CFers there into their 5th decade quite frequently.
The issue can often be resistance with so much antibiotic treatment so frequently. But, patients with CF rarely die of resistance. They most frequently lie due to lack of lung function (repeated infections over time, causing lung damage). Rarely do you hear "oh, the patient had an FEV1 of 60%, got an infection that was pan-resistant, and died a month later).
So many physicians, especially in the US, are giving strong consideration to the Denmark Way.