Yes, an original Canon 5D can work well as a second body or backup for event photography, especially if your main goal is to avoid lens changes and stay with your existing Canon lenses.

A second body choice depends on what you need most:

• a backup if your main body fails
• two bodies ready with different lenses
• a different field of view from the same lens
• similar handling and output to your main camera

If you want a similar full-frame look and familiar operation, the 5D is a reasonable companion to a 5D Mark II. If you want a different look from the same lens, a crop body may be more useful.

That said, one answer strongly suggests that matching your main body more closely is often better, because differences in output and limitations of the older body can become obvious in real use. So if budget allows, another 5D Mark II would likely be the more seamless pairing.

In short: the original 5D is a valid second body, but for consistency, a second 5D Mark II is the stronger choice; choose a crop body only if you specifically want the crop-factor advantage.