This is a bad thing for a lot of reasons. The biggest one is that Synergy is going to get the bad press when users Google "free synergy download" and download the first random binary they come across, which could contain anything. Those users are going to complain "Synergy gave me a virus" or "the installer crashes" or whatever else and hurt the reputation of the project. This is regardless of the fact that they didn't download the official Synergy binary.

The second is that this will hurt the discoverability of the software. There's no way to try it now and see what it does and PC users are used to having that ability. Throwing up a paywall is going to strangle the community, IMO.

Third, you've now stranded anybody who downloaded it free from getting updates. What if there's a security flaw? Charging people $5 for what will be perceived as fixing bugs in the software is going to create a lot of grouchy users, I imagine.

An additional one is that the idea of "we're charging because we can" is something that I think has bad optics; using an FSF essay as your primary justification for charging for "free" software is going to rub several people the wrong way. A low (percentage) donation rate doesn't translate to a low amount of income, and I find it disingenuous to try to conflate those numbers.