I've also seen Pippin Williamson, Brad Touesnard, and many other people give this advice.
Thomas states it like this:
Just don’t promise launch dates for a feature. Ever. No seriously, trust me on this. People will ask you all the time when “feature X” is ready. A good way to answer that question is (if you plan on doing it), “We’re considering this feature for a future version. I can’t give you a date on when it will be ready.
Just be honest to your customers — you don’t know yourself if and when a feature will really be ready.
Even in my own experience running a website, I've made promises that then have been hard to keep. I need this advice, and maybe you do too.