Wingback toward Middle
A Wingback Toward Middle is useful for increasing your Central Defense rating. However it will reduce your Wing Defense as well as slightly reduce your Wing attack rating on the side where the WBTM is played.
WBTM and the Wing Attack Rating
A WBTM uses their wing skill to contribute to the Wing Attack rating on their side of the field just like any Wingback. However that skill is reduced.
It is like you are playing with slightly less than 1/2 a Central Defender and slightly less than 1/2 a Normal Wingback (who contribute to wing attack).
This has serious consequences for your wing defence rating on the side where you play your WBTM. Going from playing a Defensive Wingback to a WBTM is a big change.
The wingskill of the WBTM should also be taken into consideration for maximum efficiency of the player. They are like a Normal Wingback in that their defensive contribution on the wing is slightly less than and their wingskill contribution is greater than hypothetically half a Defensive Wingback.