Flexor the Mighty!
18/100 Strength!
Yeah, I can't see the wiggle room either.
might - past tense of may - expressing possibility
if you will note that ANY spellcaster, full progression or not, MAY choose to take spells of any level as they advance, even if they have a 9th level spell slot they MAY take magic missile.
In my own opinion, I think marsupial_riot is incorrect as far as his/her interpretation of the multiclassing spell rules, both by RAW and RAI.
But, I thought people liked the "rulings, not rules" aspect of 5e, and that peoples' rulings couldn't be 'wrong' per se?
The rules for multiclassing specifically prohibit the use of higher level spells if you do not have the required levels in a class to cast them. This is the specific rule (pertaining to the case of multiclassing) that trumps the general rule for how spellcasters work.
"You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class. <SNIP> If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells." PHB, page 164
This example presented on the same page makes this clear.
"For example, if you are the aforementioned ranger 4/wizard 3, you count as a 5th-level character when determining your spell slots: you have four 1st-level slots, three 2nd-level slots, and two 3rd-level slots. However, you don’t know any 3rd-level spells, nor do you know any 2nd-level ranger spells. You can use the spell slots of those levels to cast the spells you do know—and potentially enhance their effects." - PHB, page 164.