I hardly ever shoot carbons without them....they are worth every cent they cost and the extra steps when building arrows!
The plastic thing may be fine for keeping them snug and all, but won't protect the back of the arrow like epoxying them in on a hard impact.
When shooting heavy, high performing bows bows and stumping a lot in the NE like I do...rocks, frozen stumps, The Muzzy Shoot, etc... are all ultimate tests for arrow toughness!
I foot the point end of all my stumping arrows, and that protects and makes that end about bomb proof....but....the nock end is the weak link as all the energy is sent through the shaft.....push in nocks will blow right out and the back end of the arrow will get hairline fractures. Rendering them useless unless you can cut them down and still stay tuned.
I'm sure with the adapters only pushed in with the plastic holding them, the same thing will happen....nothing bonding it to the shaft...hence ,no protection....
Now, when you epoxy the nock adapters in with a high quality, slow set epoxy(I use JB Weld)...you have a extremely strong bond that basically acts as an external footing, holding everything together.
In three years I've yet to blow an adapter out, hence have not lost an arrow to a back end fracture.
On a hard impact the worse that happens is you blow the easily replaceable glue-on nock off.
Glue one back on and your good to go.
I highly recommend you epoxy them in
These are the one's I use...
http://www.shop.bigjimsbowcompany.com/Traditional-Nock-Inserts-5-16-AA101.htm