MIL says it is time for DD (just turning 3) to stop using potty and use big toilet. She's too small to get onto toilet alone and would need supervision however she is able to get the potty out herself and use it. She is happy to sit on the potty for 20 minutes at a time which is lovely for a break! On the other hand she is quite happy to use a toilet if you take her there, and uses the child sized ones at nursery alone. Basically, I'm just lazy and using the potty is easier for me, but is there a developmental reason why it is necessary to stop and when should it be done??