I favor single position players. I play 4411 in 90%+ oh matches, adjusting tactics rather than formation against my opponents.

I'll have someone to sub my flank positions, historically like a DL/DML or DML/ML, and another for right flank. This covers the few times each season I might have red card or injury to any of those 4 positions (accepting a yellow caution out of position for a match or 2). I'll have an extra ST, GK, and MC, all single position. Only other dual position I like is DMC/DC, who will sub for a DC who can't play, and offers my only alternative formation, 4141, used rarely.

I've started a MR/AMR for last few seasons, sometimes hell score when pushed up to the more attacking position.
I've also had a DC/DL who plays DC, i tried to use his dual nature to increase his speed +DC skills, while he's faster, he's no better than my single spot DC.

I wouldn't buy a 3-positions player for my strategy.