I've never really had a problem with this regardless of edition. While i do think that the increasing number of offensive spells since 3.0 edition in 2000 has diluted the wizards' and sorcerer's main trick somewhat, you still don't have priests casting fireballs and power word kills and phantasmal killers, by and large. As long as there is still a relatively strong dividing line between clerics and wizards for spell themes, i still don't have any parties who are happy going adventuring without a wizard type in the mix.
Fireball, power word kill, phantasmal killer... maybe there's no option in 3e for those 3 spells
specifically, but there are definitely
core options for Clerics to shoot fire spells, save-or-die and similar effects.
Really, the fact that a Cleric can't ever cast a
Fireball or a
Power Word Kill preserves only a mockery of dividing. The problem with 3e splatbooks was rather that designers tended to add spells for every occasions and for every possible task to the Cleric list, often with the purpose of "covering" for some capability that the party was missing (like, "you don't have a Rogue? here's some clerical spells for finding traps, become undetectable, and stealing objects"), and perhaps justifying the inclusion by saying "well, a God is a God, can do anything so he can also bestow any sort of power to his Clerics".
The OP mentions a
real problem, that is also a non-sense design choice when everyone else in the game does
not have the luxury for cost-free (potentially) endless power increase, even if in practice it may seldom happen when you're gaming with reasonable players.
Still, it makes no sense that the designers are unaware of this, or are ignoring the issue, when it would actually be very easy to rule it once and for all. Swapping known spells, requiring paying a cost (like Wizards do when scribing), or just enforce a level-based limit on additional spells would all work, and there wouldn't even be need to playtest them.