From the Whitepaper
https://steem.io/SteemWhitePaper.pdf

SP balances are non-transferrable and non-divisible except via the automatically recurring conversion requests. This means that SP cannot be easily traded on cryptocurrency exchanges.
SP is a requirement for voting for or against content. This means that SP is an access token that grants its holders exclusive powers within the Steem platform.

I would say that the non-transfarable charakteristic makes it save against possible cheatings. You can rely that the Steem Power will not vary too much within short term. Usually comments are payed out after a certain time frame. Being able to transfer Steem Power easily would let people send around steem power like crazy to avoid potential penalties.