When an organization doesn't respond to an inquiry by a potential volunteer it's really a slap in the face of someone who sought out an opportunity to help. They may feel dismissed and unwanted and they may wonder "Does this organization really need my help? If they can ignore my offer they must be doing pretty well." If a person feels slighted then it seems only logical they won't put making a financial donation to the same organization at the top of their to-do list.
Similarly, if a volunteer shows up to an event and discovers there are more than enough helping hands that can make them feel frustrated and taken for granted. It pays to remember that volunteers usually also have paid jobs and/or children, hence they are typically busy people who are making space in their lives to help a non-profit. It can be difficult to know how many volunteers will be needed, especially if an event is new; however, organizations should at the very least offer sincere apologies, promises to get it right (or, at least, better) next time, and think of ways to show the volunteer they are indeed appreciated.
Numerous polls and studies have shown volunteers are more than twice as likely than non-volunteers to make a donation and they give more money. So it's a no-brainer that making the effort to treat volunteers well should be a key part of any fundraising plan. To learn more check out these resources:
Fidelity Charitable Gift Fund Volunteerism and Charitable Giving in 2009
Volunteering and Civic Life in America
The New Volunteer Workforce