>>it seems to me that for photojournalism, SLRs don't offer any significant advantages.<<
SLRs are more flexible.
You don't need separate finders for very wide lens, which are generally easier to focus and frame with an SLR. The Leica system's telephoto ability becomes highly stressed past the 100/105mm lens range and impossible past 135mm.
SLRs offer "wysiwyg" viewing, giving more quick comprehension of depth-of-field effects.
Not inherent to the design, but SLRs have flash syncs of 1/60 and higher, compared to somewhat shorter sync for Leica M. Ability to quickly use flash, indoors and, especailly, outdoors with daylight fill, is an important tool for photojournalism. I'm not commenting on the artistic merits of available light versus flash, only the fact that most news photographers learn to be comfortable with flash as another tool allowing them to get any image at any time under any conditions.
Sports, sports, sports! I am a die-hard rangefinder user. A former news photographer who got "promoted" into just writing, I have gone for years at a time using only my rangefinders for personal and family photgraphy. But when my 9-year-old's soccer/football team moved to a fullsize field last autumn, the 135mm telephoto just didn't have the reach. I unpacked my Nikon F2 from the closet along with 180/2.8 and 200/4 lenses. If I was trying to shoot consistent, professional-quality pix, not just family snaps, I'd probably carry another body and pack the 400/5.6 and, if I was on an expense account, a 300/2.8. Long telephotos are also essential to cover many of the standard events that are part of photojournalism, where media is kept at a distance, either at police incidents or news conferences, or where it would be inappropriate or disrespectful to be too close.
Zoom lenses. Standard photojournalism outfit today is two bodies and two lenses ... a wide fast zoom and a long fast zoom. That's really simplicity, if you think about it, allowing you to not worry about which lens to mount and just concentrate on getting an image.