Hella has published date (it's somewhere on their website I believe) attesting to their efficacy; the Canadaians also have data. The local police chief here once showed me an eye-popping statistic about how effective they are in reducing daytime accidents. I won't quote the number, but if it's even a third of what I saw it's a strong case for DRL's.

Some countries that require DRL's (certain Scandinavian countries and Canada come to mind) have a lot of twilight and two-lane rural roads. It seems intuituve that DRL's make an awful lot of sense there.

Though you don't exactly become invisible if you run without DRL's, they aid in early visual acquisition (detection), depth perception and visual tracking. In winter conditions with extreme solar glare with the sun behind you, they become very useful (as you CAN become invisible in these circumstances).

I've wired my lights so my foglights (cheap H1 bulbs) are my DRL's - sparing my tail lamps, instrument panels lights and VisionPlus capsules from the burden of full-time operation.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)