Again, this comes to the question of what your intention of building the list is; if you are planning for a long term email-marketing business or a quick “churn and burn” list system. If you are planning for the latter, you do not require a high quality list where you have to make sure that the lead is really interested in what you have to offer them.
The lists built with double-optin are more responsive and tend to take more action when emails are sent to them, which is the most important factor in email marketing success.
Some email marketing solution providers make the double-optin process a mandatory step when capturing emails. Others do not allow adding emails in bulk or importing email databases unless you make your email collection method clear to them.