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I typically prefer my beer only a bit cold, so when I buy a 12-pack from a store's cooler I typically just leave it out. The excess I'll refrigerate at the end of the night, and sometimes repeat the process with the same beer on a different night.
Does this affect the quality of the beer in some chemical way? I almost exclusively drink IPA's and personally never notice a difference, but many of my friends have commented on the habit of mine to let beer warm.
TL DR; No.
Beer flavor changes over time (hops fade away, oxidation takes hold, etc.), and this process happens more quickly at warmer temperatures than colder ones. But there are no additional chemical reactions caused by temperature changes, so warming to room temperature and re-chilling multiple times is not going to have any added effects on the beer. Assuming you are drinking it within a few weeks, you won't notice the difference with bottled or canned beer.
I think this myth took hold from left-over kegs after parties: A half-empty keg that was dispensed by pumping air into it will start to oxidize much more quickly since oxygen is being added to it. When it warms the oxidation speeds up and it tastes stale within a day or two. Keeping it cold slows that down a bit, but even cold it won't last very long. A keg being dispensed using CO2 is a different story: that will last as long as bottled beer and can be warmed and rechilled without ill effects.
In all of this, I am referring to room temperature. Beer left in a car in the sun for hours will start to stale much more quickly, even if bottles are protected from the sunlight. You won't get the skunky flavor caused by the light, but it will taste stale.
Repeatedly cooling and warming (to ambient temperatures) a beer can induce a permanent haze, where proteins and tannins bond to create semi-soluble molecules. While this can have an aesthetic impact, it does not impact flavour, aroma or mouthfeel.
This is mostly an issue in beers where the knocking-out, or rapid cooling of the beer may not have been effective at precipitating what is known as cold-break proteins. Which generally leads to chill haze (haze when the beer is cool, but not when it is warm). This generally isn't an issue with most commercial beers (especially if filtered), and is more often found in home-brewed beers.
I did some experimentation at home to answer this question. My results indicated that room temperature and temperature fluctuation had no impact on flavour. Very high temperature (140° for 24 hours) seems to create a very slight hard to define harshness. Check out my results here:
Beer Experiments: Sunlight Exposure and Temperature Regulation
Cooling and warming a beer does have an effect, but it is minor in the beginning. If you heat, cool, repeatedly many times, there will be evidence of damage and it will become staggeringly obvious! At Budweizer they gave us a beer that was cycled over a 100 times! It was shocking how many off-flavours such a delicately flavoured beer can get!
I am now drinking a beer from a keg from last weekend. I left the beer keg inside my storage with door open and it's been warm this week (100°F). Five days later the beer tastes almost same exept that the beer foam has gone which in my opinion the beer foam gives it a better taste.
