I have to admit i used to think yes. But although i have bought 3 a GK and 2 MC's its always a gamble. One of my MC's at level 44 is outplayed by another player at level 36 and age 28. Plus my GK is a scouted player and he is awful.
I often see teams with average players doing well. For instance my G/F's team is top of her league and in the final of the Champions league and she has never bothered with it. I bough a few players for it but the team is still rubbish. I set the formation 4-1-2-1-2 and left it. No moral boosters or health boosters and she seems to win every game even against teams levels above her. So i would say NO its not worth it. Infact i think the whole game is a con. Special abilities are also rubbish as the add no value to a player whatsoever.
Concentrate on tactics and formations alongside playing at least one friendly a day. People often make the mistake of thinking players need to be 99% fit for every game to maximise performance, this in my experience is incorrect. Also train your subs and reserves extra hard so that they don't get left behind.