A wither in each hand makes a high lore character into a pretty decent combat machine.True, but the big disadvantage of Spells IMO is that they are hard to use against "mobs".
Innmouth rulebook page 14 wrote:
Spells, whose rules text takes the form of “Cast and exhaust
to gain +X to Combat checks until the end of this combat,”
last until the end of the combat in which they are cast (i.e., for
multiple combat rounds until the combat is over), as long as the
investigator continues to devote the required number of hands
to keep the spell going. If the investigator stops devoting the
required number of hands, the bonus expires. Since the spell is
exhausted at the time of casting, it can only be used to fight one
monster each turn. In the final battle against the Ancient One,
however, such spells must be re-cast for each attack (and can
be, since they refresh at the beginning of each round of the final
battle). Examples include Wither and Shrivelling.