Id say if you plan on doing any really really long passages then you could buy a backup handheld but if its just cross channel and coastal runs then its not really worth it.
The Navavman is a good unit for navigating by. Depending on your time at sea and distaance you intend to cruise/travel. A Hand held is used to suplement a main unit.
If you never intend going far or only in fair weather, I would not run to the expense. However, if you intend to cruise long distances reguarly and venture offshore then a spare vhf is recommended.
