Sadly, there still exist a large number of people in the world who don't fully appreciate what Free/Libre software is all about. You're here because you're one of the lucky few who realise that it's important to know about these things. So today I'm going to try and arm you with some answers to fallacies that the poor deluded masses often cling to, regarding Free/Libre Open Source Software.
You get what you pay for, so Free Software must be crap
In today's high-consumption low-cost society, we have a perception that paying more for something increases its worth to us. However, this is a very capitalist approach. Free Software introduces the dreaded 'c' word as discussed in our article about project organisation and that often muddies the waters, particularly for big business. The main way to combat this thinking is to shift the perception of the value of the software away from what you pay for it (nothing) and onto what you didn't have to pay for (potentially millions and millions of dollars if you believe the COCOMO model.
A corollary to this fallacy is:
If it costs me nothing to get, then it must have cost nothing to create.
Again, this hearks back to a capitalist belief that monetary cost is indicative of value. In this case the trick is to get people to understand that at a very basic level it costs essentially the same to develop software to give away as it does to develop software that you charge for. The gain is in the freedom to do what you want with the software rather than trying specifically for attracting particular users.
Of course, this leads on to:
If I invested this huge amount of money, I need to charge for the software.
This one only tends to apply to companies creating free software, but it's useful to remember and think about since you can use the arguments here as backup for the other points. You only need to charge for the software if you cannot monetise the project in any other way. A number of large companies such as RedHat manage to sell enough consultancy around a fundamentally free product quite successfully. Other companies simply produce free software as a side-effect of their day-to-day business in another market sector. Free Software can be an effective marketing tool for you, if you care enough.
If everyone can see the code, then it can hardly be secure.
Some people, sadly, still subscribe to the idea that security through obscurity is a good idea. However, the obvious and effective counter to this is that many eyes make bugs shallow.
Also, if you're arguing for opening software produced by your company then Linus's Law is also a handy carrot of the form "other people will help QA our software".
But, don't fall into this:
If I give it to you for free, you are duty bound to like it, use it, and help improve it.
A lot of companies think that opening their software will immediately spawn an effective, engaged, and enthusiastic community of geeks ready and willing to help at no cost. If only that were true, but sadly those of us who love Free Software do have lives which we need to lead too. Just like a patch supplied to a project can be considered a burden as well as a gift; so can software provided to a community.
It's all written for geeks, so it's not for me.
This certainly used to be the case. Linux on the desktop is almost a running joke, indeed PC World tried to state that 2015 is the year of linux everywhere but the desktop. It's quite possible that the particular Microsoft-centric (or Apple-centric) workflows you have won't be directly transferrable to Linux-based systems, but frankly most people just use a web browser, email, and write "Word documents" which is easily done with Free Software.
If I'm not paying for support, how can I expect it to be any good?
It is very much the case that if you're not paying for something you can't in any sense have a service level agreement in place. However it is not the case that you cannot get professional support for using Free Software. Above I mentioned RedHat, but there are plenty of other companies who will help you with your Free Software for money.
What if I need to sue somebody?
Liability is a very real problem in a large number of arenas and sometimes companies really do need to know who to sue if they get sued. In this instance some of the companies mentioned in point 6 will offer appropriate agreements and warranties on otherwise utterly unwarrantied software. Alternatively you should be saying to yourself: "If I need to ask who I get to sue, perhaps I should move to a yurt in a forest and live more simply".