The Vulcan mind-meld is depicted in various ways, but most of the depictions from the TOS movie era forward show it to be a deeply personal and spiritual act, one that severely impacted the less experienced and "weaker" partner in the meld. For example, Picard was so impacted by his meld with Sarek that for a time he BECAME Sarek, and was nearly destroyed by the experience. It impacted him so profoundly that he could share that mindset with Spock. The depth of the meld is profound, and it cuts to the core of who a person is, and there is little indication that it can be used surgically or in a way that shields one or both partners from emotional, mental, and spiritual effects. To engage in such behavior with a non-consenting partner would be in every way analogous to sexual assault up to and including rape. To penetrate the very soul of a non-consenting individual, to strip bare their thoughts and to push and probe and repeatedly violate their privacy and personal identity is a terrible act.
The fact that TOS never dwelt on that aspect of mind-melds is not surprising given the era in which it was filmed and the dubious morals of its creator.
But to TNG's credit there was a serious examination of the results and impact of mind-melds - and that discussion and portrayal is what makes the Valeris "interrogation" all the more horrifically disturbing. It is - quite simply - the worst moment in the TOS films and is an unforgivable act on the part of Spock, a large black mark on the otherwise sterling record of the character.
Spock's meld with Pike is a bizarre act, seemingly done out of sheer curiosity and with little thought of what Admiral Pike would desire to feel in his last moments. It is a difficult to justify decision on Spock's part and a good example of the gaping flaws in STID.