In the right situation, this can be OK. Imagine you need to print a lot of stuff in a relatively short period of time (say...I don't know...maybe six months or less). Getting all the ink you need to do this for free* can offset the $85 purchase price of the printer. When the free ink deal ends, donate it to whomever will take it. The tax deduction will offset the purchase price even more.
* I have not read the fine print of the HP Plus contract, so I don't know if there's some limit on the free ink deal.