I think it's important to remember the Cosmere mechanics that allomancy/feruchemy uses for this problem.
When using Scadrian magic, you pump Investiture through your DNA that acts a Catalyst to shape the magic; it's the same as drawing an Aon but using your genes instead if a drawing. So when you tap a metalmind you have the right flavour of Investiture, but you still need the DNA that includes the Catalyst in your Spritweb in order to direct it.
Since Nicrosil metalminds can give people the ability to use magic, they must include a copy of that Spritweb Catalyst. So it just needs to be stuck onto your soul for you to have the ability. Obviously in Hemalurgy you need a big ol' spike to staple other people's souls onto yours. I would guess that is because you get something akin to organ rejection; since its someone else's soul it tries to reject it. If Nicrosil metalminds just have the Spritweb Catalyst, then they won't have any of the personal bits that cause that, meaning a spike is not necessary and skin contact suffices.