We use jQuery in the subscription widget here:
https://github.com/Automattic/jetpack/blob/3.5/modules/subscriptions.php#L701

However some themes or plugins change the way jQuery is enqueued, and enqueue it in the footer instead, after our function has been added, thus creating a Javascript error.

We should handle these cases a bit better.

Reported here:
https://wordpress.org/support/topic/feature-request-subscription-module?replies=1&view=all