I am a Beeminder superfan so my following statements are not criticisms.
I think that if you are a fairly long term user of Beeminder the exponential pledge schedule can be a bit bothersome if you already know generally what level of pledges are needed to get you motivated.
For example I have a workout goal and I know for a fact that $5 or $10 is not motivating enough because I have massive akrasia about physical activity. So essentially I have spend $15 just to get to a $30 pledge.
I think the pledge schedule is super useful for new users who may be a bit overzealous with the # of goals, pledge caps, etc. But if you have been at the Beeminder game for a while you generally know how much it takes to make you care about a goal.