While I understand, that seems goofy.
Balor gives 1000 XP (I made that up)
So if I fight a balor, and kill it, I get 1000 XP.
IF I fight a balor,and it summons a balor, and I kill them both, I get 1000XP
If I fight a balor, and it summons a balor, the first one runs away, and I kill the second one. I get 1000 XP
IF I fight a balor, and it summons a balor, and the first one runs away, and I kill the second one. I get 1000 XP. If later the first one comes back, and I kill it, do I get 0XP?
If I fight a balor, and it summons a balor, and it sumons a balor, and it summons a balor. And I kill them all, I get 1000 XP.
If I fight 4 balors, I get a TON of XP.
If I enter a room, and there are 2 balors there, but (unknown to me) one was just summoned by the other, do I get XP based on fighting one, or two of them?
I fight a balor, it teleports away, summons a balor, it summons, it summons, it summons. All 5 teleport back (only 30 seconds later). Do I still only get 1000XP?
Now, a 10th level wizard (as mentioned) is CR 10. But the best he can summon is a CR 5, and that is using his most powerful spell. A CR 5 doesn't do much to make the encounter worse. But a CR 20 summoning another CR20, now *that* makes for a harder encounter.