It's not so much that they are needed early, but that if they survive until late in the war their value is multiplied since you have so few. I take great pride in sinking jap TKs early on, as it just makes life easier towards the end.
In my current game against the AI, it's March '43 and I have sunk 8 CV's, 5 CVL's, 4 CVE's, 8 CA's, and many DDs and a few CLs, plus two BBs. However, it's the 5 TKs that I know will help me starve japan in the end. I can take a screen too, since everyone says pics or it didn't happen. I have only lost the Wasp and Saratoga in the process, but I know this could never be pulled off against a human player.