Ditto what pika said. MRE's are heavy because they already have the water in them. As long
as you can get water on the trail and don't have to carry it all, there are much lighter options.

If you are going somewhere without water, or where you have to pack in all your water, then
MRE's are ok. I've certainly taken them on extended day hikes in the winter here, where I don't want to carry and entire stove and fuel setup to melt snow for water, and want a big meal.