I have seen it played good and bad (No i DON'T want the JLM showing up and stopping threats in front of us... but I also don't want lame excuses why in a world with the Justice League Midnight why our problems are so lesser they don't care)
To give an example of how one can address this in a reasonable way, using my aforementioned example of Tenryu Shen, an actual NPC in my game.
- Shen hasn't actually "saved" the party from much of anything. He did one specific thing early on (vaporizing some nasty zombie/spirit creatures so they couldn't infect anyone else). He does have the power to do so, but hasn't really used it, so the party doesn't really feel they "owe" him anything.
- It's actually very important to him that his true nature remain secret. He has an important mission, a mission he is not willing to compromise upon, which he (as the party has recently learned) has a personal motive for, not just a religious or casual one. (In specific, the black dragon he's hunting used to be one of his extremely close friends, before they fell to evil, so he feels some degree of personal responsibility.) This is a huge reason why he can't just act freely.
- The party actually did several things to help him before he revealed his true nature to them. They had earned his trust, and so he wanted to eliminate the walls of deception between them. This meant the players felt they had achieved something, and made the reveal feel like a reward rather than a sudden surprise/gotcha.
- There had been hints, several times, that Shen wasn't ordinary. Just his confidence about dealing with various issues made it clear he wasn't some run-of-the-mill priest. More importantly, I had him do things at various points that left unanswered questions. So, again, the reveal became something the players wanted.
- Finally, but perhaps the most important of the bunch...the players just liked Shen before they learned he was really a gold dragon. I had feared he would be hated or viewed as a lame DMPC, but after their first meeting with him they were talking amongst themselves about what his deal might be, and after they had dinner with him and his human fiancée (their wizard/artificer ally, Hafsa), they were actively on board for the shipping.
Finally, the players later on went to Shen to ask for his help. Obviously, having a
gold dragon you could call on would be super powerful! But, on the flipside, obviously having a gold dragon you can just
summon whenever you like is stupidly overpowered...and doesn't really fit with what this character would do. So, he worked with his fiancée to create some magic items, pairs of earrings. One red, one white, for each of the players. The white earrings allow them to keep in touch with one another (limited transmit-only telepathy to the other bearers, more or less), and allow Shen to monitor their status from a distance. The white earring doesn't let him directly observe them (unless he throws significantly more magic on top), it's more like a vitals monitor--if their vitals go SUPER out of whack, he can try to do something. The red earrings, meanwhile, are a get-out-of-disaster-free card; wherever they are (so long as it is on the mortal plane), they can summon Shen to them in their hour of need, and so long as
he isn't bound or completely unable to answer, he WILL come help them. But this is something he has to use
very sparingly, because of his mission--the party hasn't used it yet and odds are looking good they probably won't, not unless they really,
really need his help.
So...they do kinda, sorta,
ish have a "guardian angel." But he's an "angel" with his own agenda and life, who really does like and value the party and the party really does like and value him. The fact that he's their
dragon friend is almost totally secondary to the fact that he's their dragon
friend.