@fanfare filing an extension also extends the statute of limitations to claim a refund by 6 months. Simply ignoring it leaves your statute at April 15 (or 18 or whatever the case may be) for that year. If an extension is on file - you get that extra six months. You also gain that six months to make certain elections (although not all elections are allowed to be made during that extension period - such as traders mark to market or the option to change from MFJ to MFS.) If you are waiting for a social security number for your child for at tax credit you must file the extension to be able to claim the credits after the initial tax deadline. If you file between 4/15 and 10/15 and you didn't extend, your credit will be denied because the SSN wasn't issued prior to your "due date". Since one can't always predict when they may benefit from this extra six months (such as extending the statute to claim a refund), then the form should always be filed even if a refund is expeected.
However, if you don't file the form, you don't extend those statutes or the time to make certain elections (granted, this won't apply to many taxpayers, but the extension of the refund statute and the ability to make things like SEP contributions is invaluable when it IS needed). The form isn't simply a method of making a payment - the payment is entirely optional - even if you owe. That's clear on the form itself, though it's also clear that penalties and interest apply for paying late.
While most of the time it's not the end of the world if the extension form isn't submitted for taxpayers owed a refund, as what most of them are concerned about is the penalty for filing late (for which there is none, of course, if owed a refund) the form should still be filed whenever possible. It's important. It's not just a form for show. Some states automatically extend without a form, but the IRS does not.
