Your statement "They could be redeemed for one cent each towards the cost of a travel purchase." is no longer true. I used to love being able to offset the travel purchases at 1 mile per 1 cent, but Capital One apparently changed the redemption formula without telling anyone. I currently have 4 travel purchases on my Spark Miles Visa Business card. They must have a table for different expense ranges because the ratio of cents to miles isn't consistent. I haven't found any web pages where the table is published. Following are the amounts of my current travel purchases, the miles required to offset them, and the ratio of miles needed to offset 1 cent in purchases:
$375.55 - 60,000 miles - 1.598
$68.00 - 15,000 miles - 2.206
$160.95 - 35,000 miles - 2.175
$163.98 - 35,000 miles - 2.134
I would think the value is closer to 1 cent. You are earning at 2 for 1 but you are spending at 1 for 2, or sometimes more.