That they strive not about words (mē logomachein). Word apparently coined by Paul from logomachia (1Ti 6:4 which see), a back formation in that case. A mere war of words displeases Paul. (Tit 3:9).
Useful (chrēsimon). Late and rare word from chraomai, here only in N.T.
To the subverting (epi katastrophēi). Old word (from katastrephō, to turn down or over), here only in N.T. (except 2Pe 2:6 in some MSS., not in Westcott and Hort).” Because of the overthrow” (result epi, not aim), useless for this reason. Such war of words merely upsets the hearers.