Nah, you didn't get an unfair advantage. If someone using a DPS meter was using it to help you improve your DPS numbers, they ONLY thing they were actually doing was confirming the number of the increase. The exact same thing could be done by purely time and visual referencing. If you view using a parser as an unfair advantage, then that'd be like saying coming to the forums and asking for assistance in a better rotation is cheating. It's not =) Just people helping people improve is all. Some are better with numbers and need this as sufficient evidence.
More so, it's just annoying when someone gets their panties in a bunch because the person didn't max out their DPS and were doing 900 DPS instead of 906 if they did "x and y" instead of "y and z". And it's because of those types that people dislike parsers. When they use them to out players and then boot them. Though, those situations are usually from DPS classes doing very low DPS because they don't understand their rotations... which you could easily tell without a parser as well).
I don't use one, but I've run into some people who do and asked them to give me my numbers so I could get a reference. I was sad to see that my tanking DPS was lower than I expected, but afterwards, I tried some stuff out and when I was measured again, I had greatly improved my number. I didn't use it to cheat or gain an unfair advantage, but as a means to motivate myself to improve (essentially getting out of my comfort zone of staying in tank stance the entire time as MT).