Over 100 Pocket novels of TOS have been published and many of these picked up on dangling plot threads, including the Reeves-Stevens' own "Memory Prime" (sequel to the destruction of Memory Alpha in "The Lights of Zetar") and "Federation" (sequel to "Metamorphosis"). Shatner's ten hardcovers, co-written with the R-Ses, have sold infinitely more copies than their regular ST novels.
That's not to say that the R-Ses shouldn't write more novels, but the Shatner novels are bought by a huge proportion of ST readers compared to other TOS.
That's not to say that the R-Ses shouldn't write more novels, but the Shatner novels are bought by a huge proportion of ST readers compared to other TOS.