1.  No reason to exclude it from consideration along with all other options.

2. Open source means the user cannot be prevented from seeing what it does and how it does it.  In the extreme, it means the user cannot be held to ransome for the price of an upgrade to retain access to her own data - as happened, for instance, when Microsoft abandoned support for Word2 files.

3. See answers above.  Media produced to ensure revenues for a closed source supplier (eg Sony video formats that require the camera to be present for installation of the codecs) may become inaccessible following a computer crash if the camera no longer exists.  But you may still choose to use the product for its other features.

4. For some users it will be.  For some it will not.  If the Open Source software is also free of charge (not necessarily true) and the costs are simply support costs, then it depends on how your software support provider chooses to price the support.  Informal support for Open Source software is often free, and you may value the fact that unlike paid for support, it is often aimed at making you self sufficient.  If you value that, it is a good deal.  If the thought of being self sufficient is abhorrent, and dependence on community goodwill does not meet your needs, it comes back to price and availability of the product and its support.

Closed source software can have advantages.  For instance there is nothing wrong with being one of those users who is more comfortable paying for Microsoft products, and relying on 'everybody else' to help you when you get stuck - or even being one of those people who resents the price of Microsoft products but cannot afford the investment to buy your way out.   Rest assured that Microsoft will look after you while you continue to buy in to their package.  If you are like the vast majority of their users, they cannot afford to lose you.

If it is your money, it is up to you how you spend it.  If you are responsible for obtaining 'best value' you must consider all options.