Right now, despite being a federal court ruling, the judgment is solely on Prop. 8 and this particular trial -- it doesn't create a precedent so that gays in Texas can sue that the Texas Constitutional Amendment against gay marriage goes against the federal constitution. At this point it only effects gay marriage in California.
If it is heard by the Ninth Circuit Court, that will create a legal precedent of gay marriage violating the equal protection clause in the Constitution that lawyers in other states (especially those in the Western states covered by the U.S. Court of Appeals, Ninth Ciruit) can use to challenge gay marriage bans in their own state.
And again, if heard in the Supreme Court, it would set the legal precedent in the entire country and could (like Loving v Virginia) force legalization of gay marriage in every state.
Now, if the courts were to decline to hear the appeal (regardless of what the Prop. 8 lawyers do) based on a lack of standing by the Prop. 8 defense team (the defendants were not able to show how the Prop. 8 ruling is harmful to them) it would not set a precedent. This would be an easy way out for the higher courts to not have to make a decision on gay marriage, especially since the State of California has stated they will not appeal the ruling.
And I'll agree a Federal Constitutional ban is unlikely, it was tried at least 3 times during the Bush administration (while the Republicans had control of the House and Senate) and failed each time. The issue now is that this, for the moment, only effects the gay marriage ban in California. If the case is heard in a higher court and upheld, it sets the precedent, potentially for the entire US, that gay marriage bans are unconstitutional. As you've stated, the defense of the gay marriage ban is weak and likely this ruling would be difficult to overturn on the merits of an appeal -- as such higher courts may not wish to set a precedent based on such a poorly defended case.