If I remember correctly, the only pokemon we receive at a level higher than 50 is Absol, and that doesn't evolve. Naturally the event pokemon we receive at high levels, which do not have the egg moves we would like, are going to be bred or put in the pc. This can be solved as stated before.
Sticking with the Absol example, one could instantly begin grinding and raising it to levels where it suffices to beat the current opponent. The same occured with the Heatmor event in Pyrrhus Pyrrhon the fire mountain in Apophyll. In general, the event pokemon are in good levels to receive, and as stated before, raising their levels is easy at any point in the game, as there are given methods you can use, no matter how much you've proceeded. The same occurs for EV training as well.
The issue on the usage of event pokemon resides on Reborn's difficulty levels. Certain monsters are more capable of dealing with specific threats, and a bit of scouting verifies that assumption. Hence, event pokemon received at an earlier level, sometimes a tad lower than the user's party, are to be used in order to overcome the current obstacle. The game is against giving you the chore of going back to grind with rediculously low level pokemon as your opponents and take this procedure to last ages, by having a scale. Each new area has higher threats to deal with, and as such, better spots for level-ups.
In the end, all that matters whether a pokemon is used or not are two factors: a. How fond the player is to the pokemon. This is the less verifiable of the two, not due to dishonesty, but due to the range it has, as evidenced by the numerous threads on "the worst", "the best you didn't know you had" etc. This is solved by having the event pokemon at a proper level, which is neither too low, nor too high.
b. How good the pokemon is in general. Giving away OU threats that perform well in-game without the reliance on TMs or breeding-chains is both encouraged, as they also belong to the previous category and actually help the player perform better against the increasing difficulty, and also skeptical, in the essence that it should not be immediately ready to dish out huge amounts of damage. Example gratia, receiving the Beldum at 1lvl is perfect, because it urges you to work for your prize and raise it up if you want to use it. Something similar is supposed to happen with the Marill line if I am not mistaken, and I am perfectly fine with it. Moving along with this train of thought, if Goomy was to be given as an event, it would not matter if it was handed away at a higher level, or had a similar case with Beldum. Find it, catch it, raise it, use it. Not too time-consuming, but it still needs effort.
Therefore, taking the way pokemon games work in general under consideration, receiving a bit higher leveled pokemon depends on the pokemon itself and makes little sense for the best of them to be overpowered from stage one.