Did this salesman have an old wife who told tales ?
The reason you 'need' a ceiling is because CFL's have a lot of ways to fail, many of them are a slow death, where it 'sort of' or 'sometimes' or 'almost' works. A swift and painless death requires heat, more is better, lots more heat is even better than that.
Anything that can trap heat, including a ceiling, helps speed the total failure of the lamp. Everyone understands 'the bulb is dead.' A bulb that works sometimes is annoying, but might still have some life in it, the things are expensive, so it is not disposed of if there is hope, even if it is flickering so bad it makes you dizzy and sick to your stomach.
If it flickers, or has that high pitched noise, and is making you crazy or 'sick', call it dead and replace it.
---- Following is more information than you want, told poorly ---
A long time ago, in the early days of CFL's making really small fluorescent ballast was not easy. Switching ballasts are borderline black magic. Tiny ones are a nightmare. Finding any electronics that tolerate heat is difficult. And finding value priced parts is a knife edge of cost/life of the lamp.
One of the failure modes of CFL in those days, when the electronics were at bordering on failure, they could not maintain the flow of electricity and would strobe. The lamp would attempt to restart, and the self-destructive cycle continued.
In non-newfangled fluorescents, this was 'flicker', a normal part of the life of the lighting fixture. And yes, strobe lights or lights strobing can make some people nauseous or worse.
CFL are smaller, and the effect is magnified, twisted circular tubes are more difficult to start, so they did it more often, no parts to replace, so because it is 'new' the only reasonable was to blame people getting sick, and warts on toads, on the CFL.
CFL's are also sensitive to the direction (base up, or base down) they are oriented. Base up is more difficult, because of the heat rising up into the ballast. Enclosed fixtures, instead of open table lamp type of application again because of heat issues. Humidity or other moisture could cause endless problems, because the lamps are not completely sealed, again a heat/air flow issue.
They use about 13 watts, while that is a huge savings over a 60 or 100 watt light bulb, it is still not comfortable to hold in your hand. The heat has to go somewhere. There are lots of parts inside and they get most of the abuse. Just send it to the giant light socket in the sky, to live happily ever after.
- Really, you're still reading this ?
If you are really interested in flourescent lights. The finer points of magnetic vs. electronic ballasts, and whatever else I understand to any degree, ask and I will do my best.