The flip side of that of course is that if the loco fails, you can just swap another loco onto the set and not lose the coach capacity. Swings and Roundabouts I suppose!
Trouble is though, I think if you're having to send a rescue loco into "an increasingly busy railway" then the damage has already been done and there's always going to be the scenario when the failed train is blocked in! As already said you need to have locos sat around along with their crews. TOCs seem to get by with units, maintenance control and their tech riders. Even depot staff if required.
On another note though, I find it's quite rare for a unit to stop and have no means of continuing, even to the nearest station to de-train and tuck away into a sidings or loop. I think the only times I've known it happened is when an EMU (with only one pantograph) has had an A.D.D activation, and that's a single point failure they (single pan units) share with locos. Edit, had a 185 brought back with no alternators so lost main res (basically needed another unit as a very expensive surrogate compressor!) and another time one ran out a fuel

. The only time I've physically been on a train that has failed was a Class 47 with low water pressure (suprised?!) Fortunately it was fixed before the line protection had been laid out...
I see no real point in concentrating all your diesel and electric traction equipment (mass) into a box at one end of the train when you can stash it throughout and have extra levels of redundancy. I prefer locos in terms of interest but they don't really make much sense for modern passenger operations.