I hear all the time about people doing writing exercises to help improve their writing or perhaps to just stay in practice. There are writing groups and websites that use writing prompts to give you something to use to spur your short goal writing. I know the Writing Excuses gives a writing prompt every week.
I find that I just don't have the time to do these. My novel is the focus of my writing time right now. I can see the value of doing some writing involving your characters to help find a voice for them or to help flesh out your world so that you understand how things work, stuff that the reader will never see, but just doing writing prompts seems beyond the time allotment I have to do any writing.
Do you spend time doing writing prompts or other writing exercises? If so, do you think it has a tangible benefit? Maybe I just need to make time for this kind of stuff.