You're probably aware of the saying, "practice makes perfect". Well, that's certainly not the whole story. Notecards is great for studying in your spare moments. However, if you have a test, interview, or other event you need to study for, random bursts of studying aren't enough.
When learning new information, you need to do so deliberately. Schedule time, as early as possible, to sit down and go through the content carefully. It's critical to let your brain make as many connections to and between the information as it can.
It's very hard to make strong connections if you're distracted and you don't give it time to marinate. Your random moments throughout the day should be used to reinforce existing connections, not relied on to create new ones.
Also, be sure to schedule dedicated time leading up to your test/event to review all of the information at once. Don't assume you know everything based solely on your bursts of studying.
In short, use Notecards to enhance your learning, not as an excuse to be lazy.