TL3 is not just meant to bridge the wide gap between TL2 and TL4, but also to slowly allow more active TL2s to get more responsibility in managing the community, e.g. retagging or recategorizing topics. Note that TL3 permissions are few compared to TL4.
I’ve been on forums where getting TL3 is a means of being promoted to TL4. Since one is active, shows responsibility in using the slightly more powerful tools given to them, the admins would be more comfortable to promote the best TL3s to be TL4s. Think about it this way: if there was no TL3s, how would you sieve off the best few to promote?