We should use relay and default tx selection rules to raise the cost of double spend attacks as far as it is easy and practical to do so. This increases the value of the bitcoin network by making it practical to use in more situations for more people. Merchants of course can't rely on them being cryptographically safe, but the safer they are, the more useful.
The argument that since they can't be made perfectly safe, they should be as easy as possible to reverse so that merchants learn not to rely on them, is an incorrect one that reduces the usefulness and value of bitcoin. Merchants will learn very quickly what the costs of accepting bitcoin payments are, and the lower they are, the greater bitcoin merchant adoption will be.