Apart from the (negligible) cost of the electricity it probably makes no difference whatsoever.
Unless you live in a mansion (or let it be known to all and sundry that you've got a large amount of money stashed away in your house) the chances of being burgled during the night are almost non-existent.
Contrary to the traditional image of a burglar operating at night (whilst dressed in a striped top and an eye mask, with a bag marked 'Swag' over his shoulder), most domestic burglaries are opportunist thefts, when somebody happens to notice a door or window open. They nearly all happen during daylight hours. (It's only 'prime targets', where there are major works of art or other valuables, which organised crime gangs might try to get into during the hours of darkness).
Other types of offences (such as sexual ones), where an offender enters a property at night, are incredibly rare and it's unlikely that having a porch light on or off would make any difference.