IMHO, I feel all friction should be removed for investors, and question making Steem Power liquid. (Though of course that would be a long term goal, for the short term 3 months sounds fine.) 9.5% is a low enough inflation rate to not require a hard lock in period, which will probably psychologically discourage investment much greater than that amount.
Even so, there should be incentive to hold Steem Power, and it could be driven by gamification of the platform. Other cryptocurrencies have thrived without any holding period, while free to play games have thrived through gamification.
Of course, a balance needs to be found between the two...