To stubbornly refuse upgrading your character space, with the ingame currency..... I cant comprehend that. Its not like 10 flower gems will hurt you in the long run, and it saves so much frustration. Plus makes things easier if you wanna stock up on some items for a certain girl. (Dragons, Ampys, Manuys).
Not upgrading Items slots I could understand, but character slots.... just see no real benefit to that. To each his own I guess, but I personally would not want to be bothered with some much unnecessary and non stop inventory management.