Could it be balance?
Blackguard caster level = blackguard level. A Rgr1/Blk9 (traded all his Pal levels) has a caster level of 9, a Pal10 has a CL of 5. In this example the Blk would be getting +3/+3 while Pal would be getting +1/+1 still.
(This is the extreme example but that is how I would build my Blk.)
Admittedly, the balance issue is rather minor & these two types of character are guaranteed to not be in the same party anyway. Perhaps it is a case of Monte (he did the original Prc, correct me if I am wrong) deciding to limit the spell list thereby making some hard decisions. Then again it may be he wanted to further differentiate the two classes and made a decision based on flavour?
As a DM I'd let a player swap in & out a good combat spell.