It's not quite as simple as that. You do need to make a "bump key" which has to fit the lock. If you have a Yale or a Union, that's quite easy. If you have a less standard lock this is a bit more secure.
The problem is that mortice locks are increasingly being fitted with euro-profile lock sections that can be bumped.
High security sections like Abloys (that work on rotating discs) or truly interactive sections like newer MTLs (where the key contains part of the lock) are immune to bumping, which like anything else might become a technique of choice for non-idiot burglars.
1. Always lock your mortice (chubb)
2. If you don't have a levered mortice lock (ie you have double glazed doors) consider a high security section. These are relatively cheap to buy and fit yourself - it's the keys that can be expensive.
3.Lay an AP minefield in your hallway
Locks are for honest people, as someone once observed. But anybody leaving their house protected by the enhanced latch system that is a Yale / Union lock is asking for a surprise visit from Planet Chav.