I'm not sure I agree with "next to nothing" as a foregone benefit: that's more playing time for Sullinger/Olynyk, and means there is no need to renounce the rights to Colt Iverson, who I think could be productive. We would need to play him to increase his value beyond what it is now. ...I don't want to do that.
Let me ask you - what more do you think he could net later, as an asset? Doesn't like beget like? And how would that asset help us? Isn't the goal, ostensibly, to build through the draft, get some cap space, and then remake the franchise into a contender? What asset does he get us that accomplishes that goal? Serious question - not rhetorical snark.
You're not obliged to play Humphries, and I don't think force-feeding Sullinger and Olynyk time with little or no ability to keep them accountable is a good thing. Humphries' value is strictly as a large, expiring deal. He's not going to be worth much as a one-year rental even if he plays well.
The goal is to rebuild through whatever way is possible. But trading Humphries right away doesn't get us any closer to this goal, as he expires in the end of the season regardless (and we're not signing anyone right away even if we had cap spaces).
What keeping him does give us, though is the added possibility that if a disgruntled start becomes available in a trade at the deadline, and you can flip Humphries and some assets for him. I won't really care about cap space in such a scenario (not to mention that a number of marquee players have recently scoffed at playing in Boston -- and that's with Doc making the pitch). Also, the beautiful part about holding Brooklyn's picks is that we don't have to commit to being horrible for years and years to anticipate getting good value in the draft.