the "official" motivation behind this class is supposed to be
making a sorc able to occasionally punch during motb.
that's supposed to be so because leveling a plain sorc up to 30 does not really bring any noticeable practical goodies for the campaign itself - especially since it is your main chara and will find himself alone sometimes and so on.
let's say a bard=30 might have a pretty much similar fate. (yet, it could be discussed ofc

)
but then again, it makes a meleer incredibly overpowered in a way that it could never be otherwise, that's why a "fake" sor/brd level (for being able to get rdd levels' sake) to a non-caster will bring way better benefits, almost throwing him beyond epic as far as stats are concerned.
but playing it as the above moptivation is also good enough.