As someone who's done some VO (and directed too); it's just a fact that some TV actors seem to need the trappings (ie set, costumes, etc.) to 'get into character' and give a good performance. With just a mic and a small room, and street clothes, some just can't get into character or punch up and 'act' the dialogue too well.
Add in that most of the time, everyone recorded their dialogue separately (Shatner often recorded his lines into a tape recorder in between takes of whatever film or tv show he was working on at the time), and it really challenges some actors to key into the moment.
Add to that that they usually have the script in front of them; will in fact read off it (and it'll sound just like that they're reading too); and if the VO director doesn't care or speak up to try and get a line the way he wants it - they'll just read it and walk off.
Especially Nimoy.

Given what I've heard from TAS, I have to wonder if the VO director was just of the mentality, "These guys played these characters for 3 years. Don't want to upset them, as long as I can understand what they say, it's fine.
There's a rather famous outtake from TAS where the director or engineer or some twerp tries to correct Shatner on the pronunciation of "sabotage" (somebody around here even quotes it in his sig line); a slightly-recent appearance on the Howard Stern Show found Shatner getting all kinds of shit over it, along with the little pipsqueak for daring to tell William Shatner how to deliver a line.
I still remember as a kid, the TAS episode with the Orion Pirate ship, and William Shatner mis-pronounced "Orion" through the entire thing; so either the VO director was clueless too; or hedidn't have teh nerve/guts to correct Mr. Shatner's pronunciation.
I'm not sure what happened there, but EVERYONE was pronouncing it "Or-ee-on", so it wasn't just Shatner forgetting how to pronounce "Orion", it was somebody's lame idea about "Here's how we want it pronounced, never mind how it was pronounced in the original show or what the freakin' dictionary says."
