It's PG application season (yay for admin overload) and I have ONE letter of motivation from a doctoral candidate (non South African) that actually tracks an intellectual journey and reason for wanting to do a PHD. Majority students simply don't tell us WHY they want to do a PHD.
The letter of motivation is not to tell us you are motivated or energetic (or married!) but why you want to do the project you list in the concept note; how does it relate to your academic history to date; your wider interests and engagements (in the labour market or activism)...
I wager that the letters are weak because most students have not thought hard enough as to WHY they wish to do a PHD in the first place, except for the high status attached to it here in South Africa, and as a way of delaying entry into a hideous job market.
(i'm actually bursting so will be more constructive, and work over some guidelines that we can share on our development studies website, over the course of december)