I agree the mini Q40 makes a good backup light for normal diving or for use as a daylight. It's kind of like the small portable binoculars that are around, they are not great compared to quality binoculars, but they are very portable and you will consequently have them with you in situations where you would otherwise have nothing.
The minimum for a proper backup light if you are doing a lot of night diving, wreck penetration etc, would be something along the lines of a UK 300 or Pelican Saber light. They are relatively low profile, simple, durable, offer adequate burn time and are relatively streamlined and snag free (if you remove the clip) and offer a very intense spot that works well for a backup or for a day light. The narrow spot also works well in muddy water where the floating particles can otherwise create a blizzard effect due to the back scatter and reduce visibility.
An Ikelite mini-C lite can also work for light duty night diving and is an excellent all round light. It has a wide beam that is adequately bright as well as a narrow hot spot that works well during the day. It is also well made, durable and well thought out.