Slides add room when you are parked in a campground or RV park. That is their only plus.
All the negatives posted so far are correct. A lot more weight (500-1,000 lbs each) and future maintenance problems. We use ours for traveling so less weight makes for better handling on the road. Keeps us well under the maximum even fully packed. We also overnight in rest stops a lot where slides shouldn't be deployed anyway.
Dumbest design trend is the bedroom slide that has to be deployed to use the bed. Worthless if you're on the road a lot.
I also forgot. We frequently boondock and sometimes that takes us down some pretty long and rough dirt roads. The rig holds together better with no slides and the extra weight to worry about.