To answer the question properly we probably need to understand what a running shoe is.
For my mind running shoes (and shoes in general) were made for protection.
In the first instance that would have been to stop the bottom of our feet being punctured by sharp objects, later it was some cushioning for protection against concrete. More recently we tried to get clever and even offer "protection" from the way our bodies naturally wanted to move and in the last 4 or 5 years, the rise of trail shoes offers protection from slipping and/or tripping on a natural surface through different types of patterns and rubber on the bottom of shoes..
Obviously, the amount of protection needed is relative to the person/distance/environment/surface.
Running on a road is very different from running on a trail. From the way, our bodies mechanics have to work to the loading of certain areas.
Running on the road has a tendency (in my experience) to lead to RSI and impact-based load and injuries. So shoe fitting tends to involve helping people stay in a straight line with cushioning to fit the person's body type and required load. We want the shoe to wear the same all over. This means small idiosyncrasies in people's gaits aren't amplified or made any worse through the deterioration of the shoe.
Most injuries from a trail are either a freak injury ie. rolling an ankle on uneven terrain, slipping and falling. Or if you are a trail convert, doing to much hill work too soon. Our feet need to be free to move, our bodies' natural proprioception needs to be able to work and we need to be able to trust foot placement.
If we think about the required shoes for those two completely different types of running then its easy to justify having two pairs sitting there.
Baring in mind that EVA (the midsole of most running shoes) does take 48 hours to come back to the way it was after compression through running and that you should be cleaning your shoes if they get muddy as well as trail shoes often having more robust uppers. You are more likely to get 100% out of your shoes rather than 80% out of one.
However, like road running shoes, not all trail shoes are created equal or in fact even for the same thing. "Trail" is a huge variable. This has more to do with the compound of rubber (soft and tacky vs firm and smooth) and the depth and spacing of lug. If the rubber is too hard, the lugs to deep and spaced too far apart our trail shoe will likely be no good on wet rocks or tree roots but perfect for wet sloppy mud and grass.
We can, however, get to specific.
So when buying your next trail shoe be sure to come and have a chat.
We offer a great range from Salomon, La Sportiva, Inov8, Merrell, Asics, Brooks, Altra, Hoka One One, Saucony and New Balance