Need or want?
Over-reliance on any single system is poor practice. Regular use is great.
Here's a real example taxing me at work at the moment. I have to put an INS in a research aeroplane for polar operations; it provides a substantial safety benefit, and a certain amount of utility.
Will we use it for primary navigation? Like hell we will - using an INS that drifts by a mile+ per hour, versus GPS with 15m or better accuracy is daft for our primary purpose. So GPS will be primary. But we still will not get airborne without the INS (but nor will we without having run a GPS model to confirm it'll work throughout the flight in our operating area).
And then we'll run some old steam navaids as a third backup.
But GPS will remain our primary navigation tool.
A more prosaic example - I fly as an instructor with a GPS in my pocket with an audible terrain and airspace warning turned on. Do I rely upon it?, no - I know my training area, and am teaching and showing my student to do the same. But I'm not infallible, and the GPS's polite warnings when I get close to some class D are a valuable addition to my SA. In that case, GPS is firmly secondary.
Cross-country, I tend to use GPS in full view but also as secondary. Why? it's less reliable, but much more accurate so allows me to fine tune positioning and make little grass airfields, but I don't let myself become dependent upon it.
Just to annoy Peter, I entered a multi-day nav competition not long ago. In my class, I had a choice of gold (without navaids) or silver (with GPS) category. I entered gold and won my class achieving more than double the points of anybody using a GPS. Yes, I was working my balls off, but I did it. Good DR still works, and I know it'll still work when every bit of electronics in the aeroplane goes tits-up.
G