Another reason for reserves is that teams/drivers don't enter in time, before anyone jumps on my back, yes i understand some deals to race are last min and on occassions cars are rebuilt quicker than expected so last min entries are then put in etc, but from years of watching Diz get stressed and getting grief off teams/drivers i can say about 70% of reserves are down to entering too late, and there are regular late entry offenders! (Diz is unpaid, unthanked and over-moaned at for his work with FF1600)
I was at Diz's house last week and he spent hours emailing and phoning the BRSCC (and I pay his phone bill!

) with different scenarios of how to get everyone on the track to qualify, so that the reserves (for whatever reason they were reserves) didnt have to do 3 laps in the lunch break behind a safety car (which as it happened was only about 10 mins) and therefore not have to start at the back of the grid with a 10 sec penalty if they had the chance to race. Diz didn't have to do this, especially as he was supposed to be working for me on that day, but this seems to happen on a regular basis, and there will come a time when the rules will be adhered to and you will turn up as a reserve and get no track time at all, because you couldn't be bothered to enter on time and there will be nobody like Diz around to sort it out for you!
I have had my rant now and wont be responding to any replies so don't bother, just remember to enter in time in future so that the organisers have enough time to rearrange the timetable accordingly