We might be a bit past this in the conversation by this point, but I don't see anything wrong with a hero (or a paragon or an... um... epic-er) picking up a power/spell of roughly their current power level. Their are plenty of ways to handle this that will be appropriate to varying campaigns, and it's incumbent upon DM's to address (or, should they prefer, not address) this in a way that's keeping with said campaign.
For instance, maybe Gariok the Hunter has been watching Isgul the Wizard casting spells through many adventures and says to him "I was initially skeptical of your ways, Isgul, but I'm starting to see the value. Do you suppose that you could teach me how to cast one of your spells..."
Isgul responds, "Well, um, if you're really interested Gariok, perhaps we could start with something simple like a cantrip. Ghost sounds, maybe?"
Gariok shakes his head. "Actually, I've been watching you cast that 'fireball' thing of yours, and that seems a bit more useful."
"But Gariok," Isgul protests, "It took me years to learn that incantation, and I required many years of study and hard work and practice of simpler incantations before I was ready for it."
Gariok smiles. "You have to remember that I'm not some village waif, Isgul. I've travelled the lands with you for some time, I've fought necromancers and vile demons, held open the rift at Shar Thorr and performed the ritual of the Endless Storm with you..." Adding, "And I've been watching you cast your fireball for a few weeks now, and I've been practicing in a mirror."
I don't think that one character picking a limited number of powers from another class has to be AT ALL unbalancing OR unrealistic. If you want it to work, you can make it work.