The way to think about it, is that for each position, you want a substitute player, and a reserve player, but multirole players can kill 2 birds with one stone.
Yeah you'd be surprised with some of the bargains you can pick up, You can get 5*, close to 6* players towards the end of the season, and late at night for between 1 and 5 tokens.
But the game all comes down to how many tokens you have, ie. whether you're buying tokens, farming them, or just playing on a 35T a season budget.
Ie/ my squad depth isn't too great, as you can see on my team showcase, but I have enough tokens to heal players with red packs in emergencies, hence why I only have 1GK, he's usually only going to get injured once a season.
But for non token buyers, a large squad is crucial, as you will have to resort to playing subs and reserves, whereas I can usually field the same squad.