There is no need to justify a slide or any other feature on a TT. Either you like the option or you do not.
Slides add weight but the also add room without adding length.
Slides increase potential problems but so do air conditioner, water heaters, electric jacks, microwave and a host of other items.
Our slide contains the kitchen and a works just fine.
Slides are mechanical devices and require maintenance like many other parts of modern TT.
Bottom line is, if you fret over possible problems, then do not get one. If you like convenience and room buy one.
Like many other TT features you buy them because you want them.