Why? What purpose could this serve, and how could it better justify the attention that Griever recieves throughout the game than R=U?
It's really simple you know. If you wanted to summon something to pit against your enemy, what would you summon? Well, you'll probably agree that pitting your enemy against the one thing he considers invincible might be a good idea, because it gives you a strong psychological advantage, no? Well, Griever is what Squall considers to be the strongest creature ever, so it is perfectly logical that when she scans his mind, she decides to summon exactly Griever.
There's no arguing about this really. Just look at what she says in the japanese version:
Ultimecia: Your feelings, I shall summon the most powerful of things
[from them]! The more strongly you feel, that will be what shall torment
you. Fufu."
It says flat out that she intends to pit Squall against what he considers to be the strongest thing ever to "torment [Squall]". The game offers this excellent and highly logical explanation without any need for "R=U" assumptions.
Griever doesn't get THAT much attention really. He is supposed to symbolise the values Squall admire, and fighting Griever is thus a symbolically potent battle. But there's nothing more to Griever than that; he's hardly payed as much attention as you seem to imply.