I don't know about in horse trailers buy friends have camping trailers with slides, numerous slides on each trailer.
I've never heard them comment that they did not open and function properly.
They all have automatic leveling systems on those trailers, period.
When you refer to spending as much on a trailer as many spend on a actual house...
I can tell you from camping years ago in a pop-up and then a pickup truck camper to open and close any door or window easily the vehicle needs leveled otherwise things either bind, get stuck or jam.
Being un-level takes the clearance distances in tracks and moves them slightly and now rubs, binds and flat-out off centered happens and stops things working effortlessly.
If you have refrigeration, cooking or such not level means they will not work...so if camping someplace your food is going to spoil...for a few hours probably not an issue except opening and closing doors and in your case slides might not be pretty.
Hence many have installed automatic levelers on their trailers so push a button and the work is done for you.
Push another button and the stairs open, push another button and the slides open out...push another button and pray your power supply is working to work all the buttons just mentioned..
Jam clearances on horse trailers never use to be so exacting to open or close the tack room or rear doors... Today now with LQ trailers, slides and what those entail, it probably does make a difference that you are leveled anytime you wish to open and close any part of the rig.
Are slides worth it...well, you take a vehicle that is 8' wide at most and near double that width, yes it can make a huge difference in comfort and amenities enjoyed...
Do you need it for going to a daily local show.... only you know the answer to that and if you truly have that amount of time to need those features or will a canopy off the trailer side and some folding chairs and cooler fill your needs for a fraction of cost...
Again, only you know your needs and wants to have.
Enjoy your research and shopping.