Should volunteers in non profits be paid or does it go against the ethics and spirit of volunteer work?

Employees are the wheels of any kind of Organizations on which it moves ahead. The expenses in running an organization therefore include the remuneration paid to the employees of the organization. Therefore even the expenses of a Non-profit organization would include the salaries & perks given to its employees. Yes, the remuneration cannot be increased to an extent which makes a non-profit organization into a loss making organization. But definitely the people working in a non-profit organization must be paid and if possible must be paid their prevailing market rates. Otherwise the objectives of the non-profit organization might take beating and the people who get supported by the work of such organizations might stand to lose. The mission of a non-profit organization is generally to provide a service. For any service industry its employees are the main ingredients. Therefore quality output can be given only when one has good efficient employees. And this can happen only if one pays the employees.

Renumeration can be used as an incentive, as long as it doesn't get out of hand. Possibly providing volunteers a travel stipend or a food stipend, depending on the nature of work i.e. how many hours put in a day, how often, whether travel is involved, etc.
At the end of the day however, renumeration shouldn't be the make-or-break factor when deciding if one should volunteer for a certain project or not. If one's heart is really into it, renumeration should not even be on one's mind.