I actually regularly use Lunala to have an easy time against the Elite Four rematches in Moon. Granted, this was the same Nebby you met throughout the game so it was clear I kept it for story purposes, because Nebby joins your team as it wanted to go on adventures.
Yeah, I felt so bad putting her in a box, kinda felt like I was putting her back in the bag haha
Because of those feelings I really debated on taking her with me...but my team was already so unbalanced type-wise I just couldn't figure out how to use her :(
I caught all the legendary dogs in HeartGold the last time I played through it, and ended up putting all of them on my team. Though normally I keep them in the PC and instead turn to the Pokémon I've raised myself.
On a recent revisiting of HeartGold, I caught Raikou by utilising Mean Look and Sleep Powder. It was only about 4-5 levels higher than the rest of my team at the time of capture, and I decided to keep it in my party. First time I've ever used a legendary in my regular party of six. Really enjoyed using it actually. Named it Shere Khan.
I think a legendary trio like this would be the only legends I would use, their design doesn't feel out of place too much, and they're pretty cool. I used Suicune in Crystal a lot, I didn't remember that before. She didn't feel too overpowered to me, but I felt pretty badass using her against the Elite Four.
I like playing with legendaries during in-game runs, because they've got awesome designs most of the time and I just want them on my team. If I intend to use one, I build my team around that legendary. I treat it like a regular Pokémon, only stronger, and not like a special advantage. They're too cool to not use.
I agree they look cool, but to me they feel so different from normal Pokémon. Not only their design but sometimes it's their moveset that is just way too strong. Plus, in the recent versions, they get at least 3 perfect IVs automatically. I also like raising my Pokémon team from very low levels, and you get legendaries close to the end of the game, in most games. But they do make me feel badass when using them! Like I said above, I used Suicune a lot in Crystal, and I almost always use Latias in Sapphire/Alpha Sapphire.