One thing that tends to make fireballs very dangerous, in many cases, as compared with summoning, is that the Mages usually have a higher level than other members of their species, often a higher level than the shield, and commonly are too strong for the character to reliably block by assuming a defensive stance.
For example, the Lich’s fireball is an urgent drop-everything-else-and-deal-with-this-now situation–much more so than anything else that the Lich does. And remember that this is about whether the Mage gets to throw a fireball at all, in certain situations.
So, giving the character a safe spot (or a few safe spots) to stand that are 1 space away from the Lich/Mage would (1) markedly alter the power balance between character and Mage–even if a different spell was substituted and (2) change the tactics that players use, and (3) likely lead to another round of Dragon Inflation.
I am wondering whether the impulse to ban the 1-space fireball was a response to another thread, where the question was raised of why a 1-space-away fireball didn’t injure the caster. I never found it implausible that the caster is not injured. After all, we are not talking about throwing a physical grenade. Instead, it is a MAGIC fireball. ‘Nuff said.