Many MTB shoes (the higher-end ones) are often tested to be just-as-stiff as your high-end road shoes. In terms of weight, typically the road shoes are lighter, however if you do your homework the majority of high-end MTB shoes end up weighing the same (w/o cleats) as a road shoe (w/o cleats).

The benefits? You can walk. Also, if you unclip accidentally (it happens) while moving and reach your leg out to keep yourself up, you're less likely to slip or damage that carbon sole