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They might be, "better" than some wooden stocks, especially the lighter, thinner ones.
A lot of wide forend wooden stocks for match rifles and stocks for bedding have been made.
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USGI fiberglass stocks are better than USGI wood stocks for general use (infantry).
They're more difficult to break than USGI wood and impervious to humidity.
Wide wooden NM stocks are heavy, and intended for matches where heavy rifles with heavy barrels are preferred.
This, too, is better accomplished with a synthetic fiberglass stock (such as those from McMillan).
Wood is more aesthetically pleasing to many people, and also what would've been found on every M14 used in Vietnam (since synthetic production didn't reach the troops before the US withdrew).
The XM21 sniper rifles utilized epoxy-impregnated wood stocks in an effort to overcome the humidity problems encountered in SE Asia.
Even SEAL teams had wooden stock M14 rifles into the 1980s but I'm willing to bet they only went swimming with the synthetic stocks (which were mostly found on sniper/DMR/M21/M25 configurations since they replaced wood NM stocks with McMillan stocks in that era). My understanding is that all weapons were stowed in dry bags for any water ops, with the exception of the scout/swimmer pair, whose weapons may be exposed to seawater (depending on the threat level).
SEAL riflemen did not typically employ M14 iron-sighted rifles except in arctic tundra and open desert areas (from the photos/books I've seen).
And it's completely possible the surviving 80s photos showing standard USGI wood M14 stocks (on rifles with no optics) were just weapons they grabbed from whatever local Navy armory for display/demo "photo op" use, and that the standard USGI fiberglass stocks were favored for "real missions" (when standard M14 rifles were actually deployed, which I believe was uncommon).