I elected to get MTB SPD's because they have several versions that can be used with or without cleats. The main reason for that being high on my priority list is that I ride year-round and have found that the most comfortable, warmest footwear (down to -4F) is a pair on ordinary winter boots. At cold temps no insole insulates the cleats enough, I've tried many.
Also I like being able to wear sandals in the summer or wet conditions over 45F. This way I can ride comfortably whatever my footwear. Some MTB shoes will need to have the lugs closest to the cleat shaved down some, I use a Stanley Shurform plane for that. That's my $.02.