For what reason does one choose to go for full cap?

I suppose full cap is choosen when having very thin sides or no sides. But how do you know your sides are too thin?

I'm going for a piece for only the top but I do know my sides are thinner then when I was not balding but I dont think they are too thin. But if they would get more thinner, would a conversion from top piece to full cap not be too hard to pull off?

With kind regards,
Pete :geek: