We all, of course, are advised to bring a spare paddle in case we lose or break our primary paddle. But I think some folks don't. Personally, I've never broken a paddle, but I have lost a couple in capsizes in hard whitewater.
So, I've always taken at least two paddles. But I've never really considered one a "spare". All the paddles I take are primary for different conditions or applications.
For 40 years I've always carried a straight paddle and a bent shaft paddle on all trips, whether day or overnight trips. When those trips included whitewater, I sometimes carried a third whitewater paddle, longer and stronger than my flat water straight paddle. However, I found that carrying three paddles was too inconvenient on portages. So, when I discovered all carbon paddles, I eventually settled on a 48.5" ZRE bent with an outrigger blade and a 57" ZRE straight paddle with whitewater blade for my extended trips. This lightweight combo handles flat water and whitewater conditions.
On day trips, I have often brought along three to five paddles. I own so many of them, so why not use them? I switch them off during a day trip just for variety or to compare their efficiencies in different canoes. On this trip, for example, I brought along (left to right) a Bruce Smith ottertail, a Brad Gillespie Free (designed by me), a Sawyer Manta single bent shaft with double scoop power face, and a Mitchell double bent shaft Leader. None was a "spare". All were used and enjoyed, providing different "water feels", propulsion forces, and manipulation characteristics.

Using different combinations of paddles in different types of canoe hulls is one of the mental, intellectual, endorphin and spiritual joys of canoeing. Not just the same kerchunk, kerchunk, kerchunk with the same paddle in the same boat every day, every year, every decade.
"Variety is the spice of life." — Casanova and Cleopatra
So, I've always taken at least two paddles. But I've never really considered one a "spare". All the paddles I take are primary for different conditions or applications.
For 40 years I've always carried a straight paddle and a bent shaft paddle on all trips, whether day or overnight trips. When those trips included whitewater, I sometimes carried a third whitewater paddle, longer and stronger than my flat water straight paddle. However, I found that carrying three paddles was too inconvenient on portages. So, when I discovered all carbon paddles, I eventually settled on a 48.5" ZRE bent with an outrigger blade and a 57" ZRE straight paddle with whitewater blade for my extended trips. This lightweight combo handles flat water and whitewater conditions.
On day trips, I have often brought along three to five paddles. I own so many of them, so why not use them? I switch them off during a day trip just for variety or to compare their efficiencies in different canoes. On this trip, for example, I brought along (left to right) a Bruce Smith ottertail, a Brad Gillespie Free (designed by me), a Sawyer Manta single bent shaft with double scoop power face, and a Mitchell double bent shaft Leader. None was a "spare". All were used and enjoyed, providing different "water feels", propulsion forces, and manipulation characteristics.

Using different combinations of paddles in different types of canoe hulls is one of the mental, intellectual, endorphin and spiritual joys of canoeing. Not just the same kerchunk, kerchunk, kerchunk with the same paddle in the same boat every day, every year, every decade.
"Variety is the spice of life." — Casanova and Cleopatra