Gets all the dex entries - Meh, I only do that Pokedex in the post-game
Rewards hard work - You're right that it's tedious. But that's not because the Pokemon are at a low level, it's because Reborn (and most main series Pokemon games) are terrible for grinding. I don't enjoy it in the least in a game like Reborn. Though it's always been very enjoyable for me in games with a Vs. Seeker. I love running around looking for people to battle again and trying to figure out the fastest way to beat a certain trainer or the way to trigger the most different re-battle-able trainers in one usage. But in most games grinding is just "go into grass, attack, get a tiny amount of exp." If grinding were faster for those who planned and strategized, it would still be hard work but less tedious (similar to how beating gym leaders is easier when you've got a strategy but becomes a nigh insurmountable challenge when you don't)
Feeling of raising a 'baby' - <3
Gets moves without heart scales - Eh, Heart Scales are a renewable resource which I am not against abusing
EV Training along the way - Much helpful here. IN my case, I'm not actually doing real EV training, just letting them pick up EVs as the battles go along. To give me a higher level Pokemon means I either have to stop my story and go catch it up to my team on EVs, or toss it in the box
Adds gameplay time - Good for people who have time to play, bad for people who
Worth it if the player likes the Pokemon - Eh, I'm not really grinding per se, so it doesn't matter that much whether I like something or not
Creates a bond - And here we go. See, I've had both types of bonding happen previously, but never with a Pokemon I just picked up for a battle. Though I've only done that like twice (if I'm gonna make a swap to defeat a specific opponent, I plan it in advance). Once in Reborn I got a Pidove for Florinia, and I don't even remember if I kept it as an HM slave or shoved it my box when I found Emolga... And in XY I caught a Zweilos to fill the last slot in my team before the E4 and replace whatever HM slave I had at the time. Post E4 I tossed it in the box, I don't think it's even level 100 yet... Even my HM slave/egg hatching Volcarona and my Thief Noivern are level 100 at this point (And I love the both of them, Volcarona almost never leaves my party anymore, even when I'm not using it)
Anyways, both ways. Back in Gen IV I was facing off against the E4 Fire guy. And the only thing I had left alive was my level 30 Tropius who literally only knew Fly (I had deleted all his other HM moves in hopes of getting some actually useful stuff as he leveled a bit) And managed to revive one of my aces and take down four (probably less, I feel like the number gets bigger every time I tell this story) of his Pokemon before being taken down. And I still love Tropius, purely because of that. I'd also had it with me for most of the story, thus why I'd removed its HMs in hopes of it becoming useful
But I've also got things like the aforementioned Volcarona who I love purely because I've had her with me for so long (I've got like 500 hours on Y, and she's been with me for at least 300) and she has yet to fail me. Even when she was just a Larvesta there to help me hatch eggs she did her duty well and slowly crept into my heart
It's not even grinding that creates a bond, it's the need to keep them with you for a while. If I could take a level 15 Pokemon and within five minutes make it match my team's level through "grinding," that'd be no different than just giving it to me at my level. It's the fact that I have to spend time with it that brings me to bond with it. Personally, I don't grind very much. I get a new Pokemon that looks cute or fun to use (EX: Inkay in Y) and it might be a ways behind my team (about ten levels at that point) but I'll toss it in the front of my party and switch-train it as a progress through the story (or just Exp. Share it if I have that) It might need a little grinding to be up to par by the time I reach the next big battle, but most likely it won't need very much. But since I had it with me and got to watch it grow anyways, and maybe it even did some Growl shenanigans to save my team from getting crushed by that one trainer and his over-leveled Whirlipede, I've grown attached to it and am excited to use it in the next fight
There's also breeding being mentioned. I love breeding as a post-game thing to do things like create competitive Pokemon and shinies, but I don't enjoy stopping my story progression for it. It's a neat mechanic, but not one that's enjoyable in the middle of the story (especially since you have to go all the way back to wherever the breeder person is to do it, instead of just throwing yourself at the next wild battle)
I'm sure there's fifty new posts since I started writing this, so Ima just post it now
EDIT: Oh, and also, I don't think they need to be at level 5 when everything is level 50. Somewhere around half the level of wild Pokemon/trainers (not sure how big the disparity between those two is) at the time would be comfortable. There's also things that don't evolve that could more comfortably be found at slightly higher levels than those that do