My company travel policy is all economy all the time, lowest fare possible.
However, I will request that they purchase Y/B for international travel in the following situations:
A) I have a highly complex itinerary (> 8 segments). In 2011, I had 6 separate trips that fell into this category
B) There is a >50% possibility of a change occuring mid trip. I loathe having to deal with a foreign carrier ticket desk when a last second change occurs. While not bad in Europe, try Istanbul where English is rarely spoken.
C) I am flying into an uncertain weather or other IRROP's situation (like a snow out in Europe).
With regards to Item C, in the event of IRROPS on a foreign carrier, I want to be as high up on the standby list for the next flight as possible which means a Y/B ticket. I do not like being low on the standby list as a *G behind other *G's and the elites of the airline I'm flying on (LH most of the time). I had 5 flight cancellations during Dec 2010-Feb 2011 where a higher ticket fare helped out on getting me the next flight instead of being stranded.
Interal in the States, it's cheapest possible as my 1K status is good enough to be high up on the standby list in the event of IRROPS.
While I fight, claw and scrap to get into B/C, my primary goal is to get to the meeting on time, otherwise, why go in the first place (well, to earn miles and hotel stays/points, etc.)