I use a lot of different formations, setups, which then require players for all positions. That is why I like double and triple position players which enable us to change formation or player's position during match without using substitutes. Recently I became more cautious about players form and noticed that most of these players can play only one position effectively. That means their average form is around 7 on one position and 5 of other (trained) positions.
So I had:
AMC-ST-MC who played good on ST (8-9), but horrible as MC (5-6), average as AMC (7)
DC-DR who was good for DC only, but had constantly 5 as DR although right footed
DC-DMC for DC (7) only, 5-6 as DMC
ML-AML for AML (8-9), 6-7 as ML etc.
It is frustrating for me to have player for 2 position, but useful on only one, just like single position players. It is even more unfair for players who train that 2nd or 3rd position. Why should someone spend 50 skill points on new position and then realise that player is useless on that position?
Similar problem is with players who are in great form, then they learn special ability and then form suddenly falls down - became useless.
Have you noticed same problem with double and triple position players?