This is not a solution, but a hack and opens up people to possible security risks. Why would this even be given and not properly solved in the code? As of right now with these settings I can easily store malicious executable code in the tmp dir. Which is a great foothold to expoit the box then. This also means any process on my box running Piwik can write to this dir.
What is the thinking behind such a suggestion by the Piwik devs? Can you please explain how this solves the problem properly and does not further open a box to a greater risk of being exploited. Fingerprinters like blindelephant can be used to find such installs. I have actually updated my local copies of its DB to start looking for that.
This type of recomendation reminds me of stuff OpenCart spews or even OpenX.