Why are you giving potential customers an excuse not to use it every month? Are you basing this assumption on any real data? An obvious disadvantage is it could

Why are you giving potential customers an excuse not to use it every month? Are you basing this assumption on any real data? An obvious disadvantage is it could drastically reduce your revenue - Someone paying $10/month will remember to use our service regularly. Someone paying $10 for a one time use may find it expensive and even if they do not, may decide to go with a competitor the next time they need a survey. Wufoo is in a similar business for the most part and are doing well with subscriptions.