I heard Sandor Katz speak this weekend (Farm to Fermentation Festival in Petaluma, California) and he said that one reason you can get only a few batches from the same commercial yogurt starter is that there are only two live strains in most commercial yogurt and they are somewhat attenuated. I suspect it's the same with commercial kefir--one reason there are thickeners and other additives. So it may work to make a batch or two, but not perpetually as kefir grains do.
Now that the op has elaborated, the question is more clear. Cold temps will not harm the bacteria in kefir, they slow down fermentation. If you keep your store bought kefir at room temp it will ferment more but at a certain point you'll start losing beneficial bacteria as alchohol is produced and/or the environment becomes too acidic. Depending on how warm it is on the counter and how long you leave it out it can increase the probiotic benefit to a certain point and then you start to lose the beneficial bacteria. You'd have to monitor it carefully to stop/slow fermentation by refrigerating before it goes too far. It goes pretty fast in the secondary fermentation (at least in homemade kefir) and produces a lot of CO2, so beware (and have fun! It's fascinating and fizzy!).
I'm not sure what you mean by revitalize. You might get a little more fermentation so that it's more sour or a little bit of CO2 production to make it fizzy, but I'm not certain the cultures in commercial kefir are really that active by the time you bring it home.
Kefir is SO easy to make at home with your own grains, you get control of what goes in it (look at all the stuff in commercial kefir--it's never just milk and kefir grains) AND its far less expensive. Grains are easily obtainable online (eBay, Amazon, freecycle, Craigslist, etc.) or from a friend.
In a word no. Commercial kefir is made from a starter and not the kefir grains that are used to produce traditional kefir. This starter contains much less variety and number of probiotic organisms. These probiotics have a symbotic relationship that allows them to continue to grow. In commercial kefir there are too few probiotics to have a strong symbotic relationship and therefore it is only able to properly culture in a smaller range of temperature 97°F to 111°F. This would require the use of an incubator.
I personally would keep my store-bought kefir cold.