The thing that disappoints me the most about TOS/5YM/23rdC books is the fact that they don't seem to have any real sense of continuity with other TOS books. Every once in a while there might be a reference to an older book thrown in by an author, but it seems to be an exception rather than a rule.
Both Greg Cox (The Rings of Time) and Dayton Ward (From History's Shadow) took care to keep their books consistent with DTI: Forgotten History, and Dayton and I both kept our books consistent with Greg's Gary Seven books. And I also kept DTI:FH consistent with what A Choice of Catastrophes had established about the full names of TOS bit players. So some of us are making an effort to coordinate.
Although I suppose I can understand the appeal of going back to a more traditional standalone approach for TOS novels, for nostalgia's sake if nothing else. No harm in having both approaches in play. After all, as Lonemagpie says, there's no way to fit all the TOS tie-ins into the 5YM, since there's simply not enough room. So it's probably for the best if only some of them share a continuity and the rest represent alternative continuities.