For me it comes down to the pokemon available.. It doesn't matter which is it and what level it comes at, if I like the pokemon I will use it.. I do this with pokemon that I find are useful in the game even if I don't personally like them.. Something like Kricketune/Pachirisu.. I used them to get past certain gyms but not after that..
With breeding the problem is that it forces me to discard the pokemon that I used as the mother/father and it's something that I hate.. I did this with petilil before the fight with noel.. Which it owned by the way.. Ame should look into Axing it.. I evolved it and with Quiver Dance and Petal Dance with the Own Tempo ability it destroyed 4 of noels pokemon.. Wigglytuff just cannot manage to hurt it.. But I digress.. My point is after breeding I left that mother petilil in the box..This happens with pretty much any pokemon you breed.. It's an issue for me.. Again it's a personal thing..
Another thing is bonding with the pokemon.. For me it comes down to the amount of time I spend with it, not how it performs.. I like greninja, despite it being a really bad starter in the game.. For the event pokemon in game, look at when bulbasaur was available, or even the chespin egg.. They may not perform really well but after grinding and using them for a while I kept them as a part of my rotational team.. The satisfaction of raising the pokemon made me keep them despite having better alternatives.. I guess it's more that I know how that pokemon got where it was.. I know a large part of it's history and its something I have shared with the pokemon in the journey.. Simply put, for me Pokemon games are all about the journey.. At the end of the day that is the most important and fun aspect of the game for me.. Grinding/investing time is all part of the journey.. When you give me a pokemon at a higher level I do have the chance to breed it, but it involves me discarding the other pokemon to gain a sense of satisfaction in raising the pokemon..
Also I find the argument about using common candies unnatural.. For one thing it's a non-cannon item, and I don't get how it would lower a level of a pokemon.. The concept of rare candies is that the nutrients/chemicals in the candy strengthen the pokemon.. For common candies it would mean that the ingredients are harmful for the pokemon, which basically means you are poisoning the pokemon.. Again this is a personal opinion..