Until recently, I worked in research at a major pharmaceutical company. Thus, I have some understanding of how the whole process works going from development of a drug, through clinical trials, to final approval and marketing. First to file would be a *bad thing* for the pharmaceutical industry.
Clinical trials cost a lot of money and, depending on the disease, can take a lot of time. It can take a while just to develop a drug to the point of taking it into clinical trials, too. Patents only last 17-20 years. If it takes 7-10 years just to go through the full development and approval process, that may leave only 7-12 years of patent life in which to recoup your investment and generate profit, after which time the generics will seriously erode your profits. Any time a drug goes off patent = huge loss of income.
Thus, as it is now, drugs are patented as late in the process as possible, so as to maximize the amount of patent life left at the end of the approval process in which to make profit. They can wait as long as possible now, due to the fact that they keep meticulous records (including lab notebooks) of every step in the process, so that if a competitor files before them, they can challenge the patent based on first to invent. With the change to first to file, they will have to file for the patent extremely early in the development process, shaving time off their already small patent-time-limited profitability window.
Clinical trials are very expensive to run (this is not a bad thing, we *want*drugs to be rigorously tested before being prescribed), and due to the relatively short sales window before the end of patent life, drug companies only have a relatively short period in which to recoup their investment. If we switch to first to file, drug companies must file even earlier than they do now, which leaves even less time to make profit before the drug goes off patent. There would also be increased paranoia (and likelihood) of the competition stealing your ideas. I can not see how this would be a desirable outcome.
There's also the whole issue of the global market. If they are already having to file extremely early in non-US markets due to first-to-file in other countries, then maybe it just comes down to reduced profits in the US. I still can't see how they would be in favor of a net reduction in usable patent life in the US, though. Then again, they may just not care any more. I'm seeing more and more of a trend toward giving up on inventing drugs in-house and simply buying out other companies to get their patent rights. If drug companies are no longer inventing their own drugs, then maybe this becomes not so much an issue to them.