Depends on where you are in the game.
My Constructs city uses all 3 training bldgs. With many of the specialty troops at 3 star and all of them minimum 2 star, I can make use of most units. I'm also far enough along there that I have all the troop producing AW's; that reduces the number of barracks troops I need to train as well as speeding up training times in the Training Grounds and Merc Camp so training those troops is a viable option.
My Woodelves (end, but won't go into S&D due to event, so working on upgrading AW's for a few weeks) city has only trained barracks troops for weeks now. Merc Camp was at 5+hrs/training slot before I got the upgrade tech in Woodelves. With the tech, bldg upgraded as far as possible, and building the Flying Academy and getting it to lvl 6 it's now at 3hrs/slot
. Training Grounds is upgraded as far as possible and it's still at 5hrs/slot. If not for the experience of the Constructs city I'd have questioned their usefulness a while ago. Here I'm thinking I could use the teleport spells maybe? Put both fully upgraded bldgs in mothballs until getting the next upgrade tech, bringing them back out to upgrade and re-assess until the training times become viable. Still mulling over the best use of those spells for me.