It's about quality and what fits the title. A game that is padded to get extra length tends to get boring and repetitive.
It also depends on the style of the campaign, to an extent. Halo (and other FPS like CoD4) are built around a fairly light narrative with almost constant combat situations and almost no exploration.
Half Life, though, is built around a bigger narrative with more exploration, puzzles, etc. balanced with combat.
Both approaches are great, although of course Half Life approach means you end up with a bigger campaign in terms of play length.
I think it should be considered too, the huge swing to MP, with many people seemingly seeing that FPS = MP and the campaign is just a nice to have, something they may not even touch. This seems particularly to be the case with the heavily combat focused titles and I don'[t think it's a surprise that today such FPS almost always go for a short, tight campaign.
My favourite FPS are by Valve, and I prefer their approach of putting out titles that are either heavily SP campaign focused (such as Half Life) or heavily MP focused (such as L4D). Their approach means a huge focus on just that aspect of FPS gameplay and is why, IMHO, their SP campaigns are superior to pretty much every other FPS SP campaign, and their MP is the equal of anything out there too.
I think Bioshock, Deus EX, Stalker, etc. are hybrids, mixing a lot more exploration, RPG, etc. with the combat. If you like that (and I do) then they're great. If you're more orientated to a Halo style campaign they're going to seem to drag at points.