The incentive is to be paid a fair amount for the job they were employed to do. The one which they saw advertised. Any employer looking to get people to do work well beyond the scope of that advertisement is taking advantage of the employee and also misreperesented the position when advertising it. If someone has a bucketload of unrelated skillsets as the OP does they should be paid accordingly if then undertaking that work. Granted the company may be able to negotiate a discounted rate but they should certainty not get it for free.
In my book they can renegotiate a contract for the employee to do the work, they could hire a third party to do it... or if they got shirty the employee could just find another employer who valued their skillset as it ahould be valued.
The employee has no vested interest in being taken advantage of... their incentive is to be paid a fair rate.
On the other hand, there are some very happy and successful people who worked for me and showed skills and willingness above and beyond. As to the issue of misrepresentation, any position that does not change is either very low level or in a stogy organization. Smart employees work to see positions change to their financial and or person benefit and, failing that, look elsewhere.
Huntersdad no only added value for his organization and made us of his gifts but also built some associations and took the opportunity to find value in the work he is doing. A work to the rule person would have been left out in the cold.