The biggest deterrent to filing a PCT application is the filing fees, which will cost thousands of dollars – and this does not include the legal fees for having a patent attorney prepare and file the application on your behalf. In some cases it is better to file directly in each nation than to pursue a PCT application first.
In other cases, however, the long-term patent costs are cheaper going the PCT route. Often times, smaller patent offices use the PCT written opinion as a first office action. Canada often uses the PCT written opinion as its first office action on the merits (sometimes verbatim). Other small- and medium-sized patent offices have a similar practice. (Note that the EPO and Japan are less likely to do so). Using the written opinion as a first office action can be a great advantage in these countries since you will have advance notice of relevant prior art, patentability issues and more time to develop a workaround solution or arguments to refute the examiner’s opinion.
Why else is this useful? If you are proceeding in several national patent offices, prosecution costs can add up quickly. If you have a response that can be used in several jurisdictions with only minor revisions for local rules and formatting, you realize a huge savings – usually thousands of dollars per country.
So, while you may incur steep PCT filing fees, the advance opinion on your patent claims afforded by the written opinion is generally well worth the initial cost, since you may find reduced prosecution costs that make the overall patent costs must lower than having filed directly in each country.