A lot of this discussion (and with some an argument) I think needs to be put in context. I know those who do not secondary at all and it works fine for them and its true that many say it is not necessary as I believe it is not. However, everyone brews differently and has a different process that works for them. To suggest that a secondary isn't good for anyone simply because it isn't good for your process is a little short sighted in my opinion.
I've taken beer straight from the primary to the keg and from the primary to the secondary and I've come to the conclusion that I prefer the secondary. This is because in my home brewery with my equipment the secondary makes a great intermediate step for the beer while it waits for an empty keg and frees up the primary for another batch. The secondary also helps with clarity as well as getting that gravity down that one extra point or so.
But the main reason I secondary is because I primary in plastic buckets, not glass carboys. I only have five gallon carboys but seven gallon primary buckets. The buckets are also easier to clean and versatile as I use them as measuring devices during my brew day. I like the ease in use and cleaning and the extra space for when I underestimate the amount of wort I've produced as well as the fact I shoot for six gallon batches to compensate for the beer loss due to trub and spent yeast. I couldn't make a five gallon batch if I primaried in a five gallon carboy. I also don't worry about breaking them as they get moved around a bit during brew day and are subject to some quick temp changes, something that would crack a glass carboy (I'm speaking from experience).
However, the bucket is permeable so when the wort becomes beer and the yeast settles to the bottom the plastic stands a much greater chance of letting oxygen get to the beer than the glass carboy. The perfect solution is to rack the beer to the secondary. As far as beer loss goes, if there is any left in the primary after I move the beer over I will bottle what's on the bottom of the primary. I don't like to lose beer so bottling whatever is left on the bottom of the primary after I have my five gallons in the secondary is a great way to store the beer for posterity. I only have three taps so there are only three beers on tap at any one time but I have all kinds of bottled homebrew because I've started this practice.
I don't worry about contamination either. I keep everything really clean and do my best to give everything in the area a good cleaning before I move anything from something to something else. Like Preston says, once its beer it can do a pretty good job of taking care of itself.
I've gotten to the point of where I really enjoy the day where I move the beer from the primary to the secondary (as well as bottle a few beers) because it gives me a chance to try the new beer and give me an idea of how well (or bad) things are going to be. So I don't see it as "more work" its "more fun" to me. This is my hobby after all, if I didn't enjoy the process I'd go to the store and buy beer.
I'd also be willing to bet my next pay check that when I move the beer from the glass carboy to the keg, the carboy is about a hundred times easier to clean than if I had used it to primary in.
Finally, as alluded to earlier, often times I'll make a beer and it will become ready but there won't be an empty keg or I'll have a beer ahead of it in the queue. Often times beers are going to be asked to sit for 5-8 weeks and every once in a while even longer before they get to the keg. I'd much rather have that beer off of that initial yeast if its going to sit for that long. Not to mention how long I'd have to wait to brew another batch if I tied up my primary buckets that long.
So, in my home brewery the pro/con list favors the secondary despite the fact it might not have much effect on the beer, the secondary has a huge effect on my process.
$.02