Lead Conversion Permissions Independant of Account Creation Permissions
In order to convert a lead, the user must have the permissions to create a new account. It is not possible to convert a lead without the permission to create accounts. This causes a break in the lead process if users are not permitted to create accounts.
The permission to convert a lead should be independant of the permission to create accounts. In this case the user would only be able to convert a lead into a contact and/or opportunity attached to an existing account.