Aye - but.... the funny thing about the whole recumbent/visible debate is not that there are times when you can be out of someone's sight (newsflash!) but that for some reason people are comfortable with *themselves* being out of sight but can't seem to understand why I am quite happy to ride a lower bike.
For example, 99.9% of people out there will happily ride downhill at 25mph+, in the bike lane in the door zone towards a notorious t-junction where drivers use the lane to nose out for a look - and the junction itself is obscured by closely parked 4x4s with tinted windows - so nobody can see them coming. I don't scream at them "WTF you are completely invisible you reckless w**ker" (although I often feel like it!)
But unfortunately, you'd be amazed at the number of cyclists who come up when I'm waiting in a queue and say something like "you really are quite invisible" with a shocked and passionately quivering voice. I then continue to sit in the queue, watching them scrape up the inside of the left-filter lane to get between some Lothian double-deckers to the ASL. The queue pulls away, mysteriously everybody manages not to run me over, and I pass them shortly, still riding in the traffic stream as they bump along in the gutter casting fearful glances over their shoulder for the next car to rap their elbow...
I think it's quite fair to say that you can get yourself in more trouble riding a recumbent badly - for example, if you make a habit of filtering up the side of left-turning HGVs there really is no chance that the driver will see you, but they just might see the helmet of an upright rider. However readily I agree with that, the fact that ~80% of cyclists killed in London are A) on upright bikes and B) alongside HGVs indicates that the prudent decision on either platform is just not to sit alongside a left-turning HGV!
I haven't ever really thought about riding alongside other riders. Whenever I do it I haven't experienced any problems, but perhaps I am keeping enough separation that I am still in sight at all times (plus I prefer to ride on the outside, so we get some more space from passing cars - and that's where their attention is more likely to be).