Either way, you could see this as an issue. Some would prefer gravatars both on the recent posts and recent comments widgets. Others, in concern of spamming and sharing a public blog, would like to keep it clean.
Best solution seems the option to opt out gravatar use in the recent comments widget and opt in within the recent posts widget. It results in a coherent lay-out and maximum control.
This is a concern that can be dealt with on wordpress.org, so I hope they adjust the user-friendly wordpress.com likewise.