There are a number of ways records can be inserted into objects. Use of DML via Apex through your own code is of course one. Here you can of course determine how many records are inserted at once. However the Force.com platform is a very open platform. There are numerous API's (Enterprise, Partner, REST) and also Data Loader tools (utilising these API's) that can also insert records into your objects on mass.
So the possibility still exists for bulk inserts, even if your code does not give rise to it. So as you say it is good practice, but if you have to consider avoiding bulkification you better be sure that all potential 'clients' of your object are not going to take any of these other routes and catch you out.