At the moment, mining profitability for LTC is in, or close to being in the red for most miners. Come the halving, it would not be surprising if some miners decide to stop mining LTC because of the economic disincentive thereby leaving a pool of rentable miners for an attacker. The miners would be incentivised to rent because payment would be up front. The combined effect of this would be am increase in the ease to launch a 51% attack in terms of availability of hardware and decreased 'difficulty' due to loss of hash power.
Wouldn't it be better if block rewards decreased gradually instead of a 50% drop?