The opinions are mixed on whether the compression plug only serves to anchor the top cap bolt or whether it performs a vital function, supporting the steering tube against possible crushing by the stem. If you want to support against stem clamping forces, some models that use an M6 clamp bolt can be inserted deeper with a longer bolt. The FSA style with the threaded top cap can't.
FWIW, I once did a test on a scrap piece of carbon steering tube. I installed a stem and tighten the two M6 bolts as hard as I could with a long handled hex wrench and it didn't hurt a thing. Occasionally, someone will crack the top of a steerer when the steering tube is 2-3mm below the top of the stem and the top bolt is way overtightened. If a steerer cracks under any other situation, it was poorly constructed.