Personally I would never add any glue or latex to the back of my rugs. Why? you may ask.
First of all if you execute a good piece of work there is no need to contaminate the back with any type of clagham!. A well hooked rug or proddy mat will last a good many years.
The next reason there is no guarantee the loops will stay in place and if any loops do pop out it would be near impossible to repair trying to hook loops through a latex barrier.
Your rugs need to be flexible, floppy and bendy not rock hard.
And glue/latex can be expensive.
I have heard stories from the past where people made a glue from flour and water and covered the backs of their rugs with that. I have made clagham that way to paste onto wall paper that has peeled off the wall in places lol....
Rug makers for many reasons have probably ladled glue onto their rugs one reason to make the rug non slip.
For me I don't want to waste my time and money with glue.