It frustrates me to no end that it is harder to make yourself temporarily strong than it is to make yourself permanently strong in Ars Magica.
IMO however the primary situation in which you would want to make a spell that temporarily increases characteristics is if you want to make Sun duration version of the spell and enchant it into a magic item that permanently boosts the characteristics of the user of said item. In this case the high level of the spell turning it into a ritual is less of a problem, however getting a lab total sufficient to enchant item will be a problem, again because of the high level of the spell.