Why are they called "Tier 16.5 maps"?

I expected a memory influenced map to drop in like tier 15 or 16 maps due to that designation, but I just got a T3 "T16.5" map.

Is it because they're not worth it unless they're T16?

Because they share a lot of characteristics with T17 maps. Generally they're easier than T17 maps, even when fully juiced, so they're somewhere between T16 and T17 - hence the .5.

Because they are not t16s or t17s, but t16,5s.
It's not their official name though. They're basically "originator maps" and whatever tier they happen to drop as.

because they are T16 maps but with the much higher value T17 mod pool included

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Ondrashek06#5795 wrote:

I expected a memory influenced map to drop in like tier 15 or 16 maps due to that designation, but I just got a T3 "T16.5" map.

Is it because they're not worth it unless they're T16?

It is not worth it to heavily invest in them if they are not tier 16. So people juicing their maps call them t 16,5 because they jsut run the t 16 memory maps anyways and their mods make them much better than normal t 16.

However, there is nothing wrong with enjoying the higher droprates in your t3 memory map while you are on the way to the top. Just don't use any expensive scarabs ;P