It does make you wonder why they were included at all on the SCSM table then.. unless casting the spell is complementary to something else.
The passage at the base of p.139 (RMSS) is vague enough to allow either "This gives a modifier to a subsequent related action on the part of a character. Note this modifer may only be applied to an activity that is both immediate and integrally related to the manuever that yielded this result". But follows it up with... "Note: The GM may determine that this modifier is inappropriate to a particular subsequent action, whereupon it should be ignored."
Personally, based on this, I'd be quite happy (with no quibbles) to accept it either as a bonus to the casting check on a following spell if a similar spell is attempted immediately afterwards OR as a modifier of one of the tables used as successfull result of casting the spell.
What this would mean is that spells requiring use of the SCSM table (opposed to those cast automatically) will usually provide bonuses or fail completely... perhaps if the GM is concerned about this a simple -20 applied to the result of spells cast requiring SCSM, these allows for a more approriate range of results, with a small bonus for those casters exceeding 176.
However, I'd be more in the favour of the "Risk brings reward" school of thought and that using the SCSM at all is indicative of a degree of "experimental on the fly" casting... where the effeciency of the spell can actually be increased by avoiding the "tried, tested and dependable" route...