That would be not offer any real protection against someone going back on their word because the stake can always be moved to other accounts to vote. In the case of exchange accounts which are normally kept liquid, this can be done instantly, and in the case of exchange accounts that are now powering down, they will soon be able to do the same.

There was a good reason that the soft fork restricted transfers as well as voting, because that is the only effective way to guard against the stake voting, as well as guard against the stake being powered down and whisked away, leaving nothing but empty words and broken promises in its place.

Witnesses who ran something so easily circumvented and insecure as a mere vote block without a transfer block would demonstrate lack of understanding of the relevant blockchain mechanics and would certainly lose my confidence.

The ninja-mined stake needs to remain where it is so that there is continued transparency and the community can continue to exercise oversight against bad actors who do not do as they claim they are going to do, and instead attempt to just pocket the funds and run off, as we unfortunately and carelessly already allowed Ned to do. Stakeholders and witnesses are currently in a position of "Fool me once, shame on you, fool me twice shame on me."

Of course, it can be withdrawn and spent at a reasonable rate as long as funding to develop the ecosystem is maintained (as Steemit did, albeit even then incompetently, at least up until the sham "layoffs"), and as Dan mentioned in the post, SPS can be part of that.