What can we learn from the story of Eldad and Medad? In no particular order and with no infused wisdom:
God keeps His word, and then some. In this case, His word was that He would bestow the spirit of Moses upon the seventy elders whom Moses was to bring with him to the tent. God exceeded the letter of His word in bestowing the spirit even upon the two who weren't with Moses.
God works with His friends. Why did the spirit come upon Eldad and Meded? Because they had been on the list of elders Moses had drawn up. If they were good enough for Moses, they were good enough for the LORD.
The Spirit blows where It will. We don't know why Eldad and Medad were in the camp when they should have been with Moses. It doesn't matter, though; whether they were too infirm or too afraid, God sought them out and made them His prophets.
Your charisms aren't about you. Moses understood that; Joshua didn't. (And, considering the fact that Moses was the humblest man on earth, we might wonder whether Joshua was jealous of Eldad and Medad for the sake of Moses' status, or for the sake of Moses' aide's status.)
Charisms are good. They're good for the people who receive them, and they're good for the whole people, for whom they're given. Since they are good, and since God is good, He is unstinting in bestowing them.
Moses gave a prophecy of Baptism. "If only all the people of the LORD were prophets! If only the LORD would bestow his spirit on them!" And what do you know? They are! He does!