Yes, it is about having the pretty color. No other reason to work so hard at it to get a certain look. Just because you pick a good bloodline and what addtional traits does not alter that fact. Not that there's anything wrong with wanting a certain look and breeding for it. That is certainly a horse breeder right. That's why they ultimately creat breeds with that look or color and registries for them. There's now shortage of them. Appoloosa, American Paint, Palomino, even the Friesian has a strict color standard (black) that they like to maintain. And if owning a black horse is someones desire the Friesian would be the breed I would point to.
No one breeds to a Friesian with the hope of getting a champion jumper. Friesians weren't the bred for that. I've never heard of anyone breeding a Friesian to a jumping breed in hopes go getting a champion jumper with some Friesian characteristics. Breeders will use them to introduce certain Friesian traits to their prefered breed, or visa versa, introduce other breed traits to Friesian. But to say that it's not good to breed Friesian blood to jumper, just because you think it will come out as a mutt is about as logical as thinking that breeding good blood to get a certain look or color will always work (it doesn't). Don't care now good your blood lines are. When it works you keep breeding with those offspring to keep the trait and continue breeding those trait holders to hopefully establish it. The ones that fail are excluded. No big secret there. Same system is used for all domestic animals.
You can find a great many incredible offspring from Friesian and jumper cross bloodlines. The breeders intent was not to create a champion jumper with Friesian traits as you seem to think. I dare say that most people who own breeds known for jumping don't always jump them. Just as most people who horses known for working cattle, don't work cattle with them.
There are already plenty of great jumping gaited breeds. Can't imagine any breeder that has used Friesian blood for that purpose. People who want to breed champion jumpers are not going to add a non jumping breed to get improved jumping stock. Breeding for traits is not always about what either horse might be best known for.
Indeed, the only reason I have Friesian blood in my mares is for more obscure Friesian traits the heavier bone and Friesian personality, but that was all that mattered to me. Their ASB blood line already gave me the incredible gait I wanted. The fact that Saddlebreds are also good jumpers is just extra characteristic of there bloodline, not something that mattered to me. I wanted disposition, bone, gait and size to fit my needs. I got perhaps a bit more height than I wanted with one, but that's ok too. She can carry more.
Can't say what most riders in Slovenia get horses for, but most of the of the riders I know aren't interested in showing. Many like to have a nice looking horse, but the things that tend to be more important is the horses peronality and can the horse do what they require of it. If it can, then it's a lovely horse. You could give me an "ugly" horse and as long as it was reliable, trusting, with a sweet disposition, easy gaits and could carry me and my gear for 50 miles a day for up to 5 consecutive days with ease, then that's a beautiful horse. If I had a national dressage champion that couldn't carry me more than 25 miles a day for at least 3 days without needed a 2 day break, then it's nothing but a pretty (well trained) horse that I'd sell for what I could get, because it's of no real use to me.
There are breeds (e.g. AQH and Mustang) that were Heinz 57 mutts, before a standard was set for them. Someday these Friesian/TB/ASB/AQH/TWH/etc... mutts could become very popular and widely loved, and defended breeds with their own special traits an abilities that people may use improve or create other breeds.
I certainly hope this country never stops it's continuing history of cross breeding to creat improved breeds. We've created quite a few in 300 years (by design or accident).
e.g. Appaloosa, Saddlebred, Standardbred, Morgan, TWH, American Paint, various Mountain Horse breeds, AQH, Florida Cracker, March Tacky, MFT, Mustang, and many more. Even ones created during our early history and have now vanished like the American Horse, and the little Narragansett Pacer (our ultimate foundation breed....I'll brace myself for the Morgan lovers reply to that
) ), which became a breed and then vanished as a breed in the space of about 150 years, but who's heritage continues in the wonderful gaits it passed on to all the many gaited and some non gaited US breeds (via many other cross breedings) and to the Paso breed of our southern neigbors. The Pacer is an excellent example of why cross breeding is a good thing. Look at what mixing various breeds with it's extended offsprings bloodline has given the equine world.