well, it really can be pissing off at the beginning, but in truth it really helps out for the longer poses later on. these short tests help you out in learning to define the important parts of the human body, the gestures and movement present in the pose, which is what really matters in figure drawing. Long poses are meant only to work out the details, but trust me, it will be better for you to make hundreds of 2 minute drawings than a couple of hour long poses (if they ever are that long :p )
I did only short poses for about 4 months before trying some serious long ones. Theres a lot you need to get from the short ones otherwise your just gonna be spending a long time working on a drawing with serious problems in the long pose. Figuring out the exact spatial positioning of the parts of the figure is much harder than it sounds. I think the ball and cylinder excersizes are to make you think especially about that.
BOth are valuable. You do a quicksketch everytime you start a layin for the long pose. THe quicksketch helps you understand gesture and proportion which carry over into the long drawing. Quicksketches also help you look and analyze more since you have to make every line count. To get better at quick sketches, you have to slow down. Then when you get to the longer drawings, you have more time to focus on your rendering, lighting, values, form, indications, edges, etc. in addition to your gesture and proportions.