100% agree, it was very tempting at the start to _not_ charge for EventHQ but in the end I charged from day 1.
Downsides are that EHQ have grown more slowly than perhaps it might have done if we had not charged. This might also be an upside as fast growth brings its own issues, esp. if you don't have any revenue.
Upsides are that: (i) I had revenue (yay!) from day 1 and profits sometime thereafter; (ii) my customers are committed to EventHQ and have been very quick to feedback on areas where I can improve. This has sometimes been painful but has definitely helped guide/prioritise development.
Non-paying customers can provide feedback as well, but I tend to find customer's who are willing to pay are more thoughtful and willing with their feedback.