Everything is known through comparison. At that moment, it seemed like it wasn't a deciding factor.

Well, it still isn't much of a deciding factor even now. More precisely, it decides no more than other irregularities in the spawn.

When Mullich and the level 4 specialists were brought back into the game, the reasoning was pretty much the same.

An imba is something that creates a clear skew in gameplay in favor of that imba. For example, diplomacy - under no-rules settings, everyone tries to dig up or level up a diplomat; the presence or absence of diplomacy on a hero is a key factor when choosing a starting faction.

The Grail does not create such a skew; there is no rush for the Grail, no discussions like "Grail is far => restart" or "Grail is on respawn => we play any crappy layout" are observed. In many cases, digging for it is even detrimental - the lost days of tempo are not guaranteed to pay off.