One of the biggest reasons they add diesels to main line excursions is the dynamic braking that the diesel can provide. Keeping the brake shoes off the steam locomotive driver treads
extends their lives significantly, and this also benefits the brakes on the passenger cars as well. Reducing wear and tear keeps the costs down and allows main line excursions to continue. Range extensions and tunnel safety are also added benefits. Montana Rail Link won me over when they added an F-45 as the diesel, maintained the passenger train lines and made the diesel helper much less noticable. Also, having the diesel along for insurance keeps main line excursions welcome on class one rails as the SP&S experience noted above pointed out. Having a steam locomotive crap out on a busy main line often means you won't be allowed back any time soon!!