That's true. However, directed spell attacks are still affected by a number of factors (range and cover, among others) which still provide an incentive to develop the skill. Additionally, the intrinsic DB of opponents tend to increase with level as well (especially because of magical means).
DB doesn't increase as much as OB does, though, or shouldn't IMO. Also, not all DB actually is useful against a directed attack spell; in fact, most of it is
not, as RM2's raw only define the QU bonus and special spells or items to modify the EAR rather than the full DB.
Also, I
did mention a need to develop the directed spell skill to a certain level: I never even once contested this fact. What I contested is to compare its development for a spell user to the development of a weapon skill for a fighter. The latter needs to develop it to the highest possible score whereas the former only needs it to develop it to a good score, which isn't even quite high: even reaching 50 OB in a directed spell skill would be enough in many situations.
And I'm pretty sure the spell attack variability is mostly useful for the low-level bolts (Shock Bolt being a prime candidate, since it can be raised to near-unholy levels of concussion hit damage if you have a DS skill high enough to reach the double digit hits on the attack result. As far as I remember, Spell Attack Variability raises the effective spell level to the number of PPs you spend, so using it on a lightning bolt would require quite a high-level character, while using it on a shock bolt is available to the masses.
It also raises the
damage inflicted, meaning that, once one reaches a directed spell skill high enough to score say, 5A damage, one doesn't need much to increase one's OB, as multiplying the damage through the spell attack variability is a good enough tactic. Sure, it works better with low-level spells, but it means that if one chooses such a tactic, one doesn't then need much to develop a directed spell skills for later, higher-level, spells.
For instance, the Warrior-Mage, who is plagued with just not enough DPs to develop all of his directed spell skills most of the times, may just choose to develop the shock bolt, forget about the other bolts, then use the SAV rule to increase his damage instead of the critical inflicted. As such, the rule gives an incense not to develop (other) directed spell skills over the lowest level spell.