In my experience, do you NEED a rad cage. No, at least not when the system is all nice and new/unworn, and assuming you have some patience to set the system up in the first place.
However, getting the RAD cage makes the system easier to set up, more resistant to wear / dirt and nicer feeling (shifting is more linear across the cassette.
I ran without the Rad cage for around 6 months, and as the components wore, the shifting got noticably worse, and i would get the occasional mis-shift under really dirty conditions. I then fitted the Rad cage (without replacing anything else) and the shifts got noticably crisper and it was easier (less force on shifter lever) to select the larger cogs at the back.
So, in summary, of you can’t afford to get the Rad cage, don’t fret, you can manage without it. if you’ve got the money, however, it’s a worthwhile small additional cost to return your wide range system back to pretty much OEM 10spd shift quality and robustness!