I think the majority of students are told that starters and plenaries are very important. Starters help introduce themes, aims, and objectives for lessons (as well as a good way to recap).. Plenaries are there to conclude the lessons learning and should refer back to the objectives set at the begining of lessons and should make students aware of what they have learnt during that time. Ofsted are also very big on starters and plenaries! Im not sure why ten minutes (Im only training tooo!). I assume it's just because that is a good amount of time to warm up and conclude the classes learning. Every school I have worked in have used to starters and plenaries- it's strange yours haven't! I would just try to make sure you always do a starter and plenary in your planning, perhaps talk to fellow course mates for some ideas if you arent seeing practical demonstrations. It would be a bad habbit to get into not to do them. Maybe visit other teachers in different departments to see if they use plenaries and starters and how they use them. I hope this helps! As I said I'm only training too so this is just my understanding of them. However I do know that I would be in BIG old trouble if they weren't in my lesson plans!