Originally Posted by
UA_Eagle
Also it allow the airlines to offer fewer miles to the flyer, since they count these flights as one "flight", despite having to change plane type and gates. On the other hand with some airlines (like UA?), it's possible to get upgrade for the whole "flight" with fewer upgrade certificates than otherwise, if the flights has been counted as 2 separate flights.
BUT you only get the upgrade if it's available for the entire flight (both segments), at least on some airlines. If only one of two has an F seat free, you won't be in it. There may be ways to work around this (on US there is a way), but it's not easy.
This really bites on airlines (CO, US, DL, NW, sort of AA, ...?) where upgrades are free for elites.