'RNG abuse' is not a naming convention I like. I would prefer it if people referred to it as RNG manipulation. The use of knowledge to both speed up breeding/capturing Pokemon and getting the best IVs is certainly not abuse.
Games are put out there, and people take them as they are presented. They play the games, they complete them, and shelve them when they are done. For some people, the way games are made are fascinating, and understanding how a particular routine or AI behaviour or programming part or whatever gives rise to people who will explore games after they are released. I suppose those people can be referred to as hackers.
Now, by hacking the games, the knowledge of how the game's RNGs have been reverse-generated and understood, and now it is almost common knowledge, at least on the Smogon forums, and is also widely used on other forums for breeding/capturing purposes, to get the best IVs legitimately. The word legitimately in this case should be more accurately defined as 'using the way the game generates a Pokemon to manipulate the RNG into producing the best IVs possible'. No outside programs used to influence the RNG should be used, although it is reasonable to use external programs like RNG Reporter to find the seeds and other information that will generate the Pokemon you desire.
This use of external methods to produce Pokemon that people want for competitive Pokemon battling should actively be encouraged. The game generates Pokemon normally in a certain way. Using the way the game is designed to generate the Pokemon you want is not only an intelligent thing to do, it removes a lot of the problems you get from hacking, or using external sources to modify the game to give you those Pokemon. With RNG manipulation, you are not changing the game in any way, you are just influencing the outcome to one that you desire. Knowing how the game works is not a bad thing, and I'm surprised at the general feeling that is still out there that this method of generating Pokemon is a bad thing.
It is very simple to think about, really. RNG manipulation is a huge time saver, which is its main appeal, there is a lot to learn sure, but once you understand how it works you can easily breed a flawless Pokemon from scratch in an hour or less. Not only is this a huge time saver, but it gets you to think about how the game is programmed, to a limited extent. Using how the game works is not cheating, nor is it abuse. It's just how it was programmed. Why would you prefer to be ignorant about something, when you can learn the truth?