Also very useful for the Modern Orthodox Jews who have "normal" jobs in big companies, where taking clients out for meals is commonplace.
You can take someone out (or be taken out) to a place where the food is as good as any other restaurant and, while expensive when compared to "traditional" Kosher restaurants, often isn't much more than a top-class regular restaurant.
London's first restaurant like this is called Six-13 in the West End. New York has loads.
DejaMorgana points out (as I knew... really... honest...) that 613 is a significant number in Judaism, as it's the number of commandments, positive and negative, that we have. The other significance of it is that it's 19 Wigmore Street (19 = 6+13). Hey, it's loose connections galore!
I also forgot to mention that the Bevis Marks Synagogue, on the edge of the City of London and the East End also opened a smart restaurant. It's sort of stealth-kosher, but being within the Synagogue's complex does destroy that idea. But never-the-less, for people who work in the city, it's a good alternative if you need a decent client lunch.