the multi purpose rotary tools just generate too much friction for pla.
The little battery powered engravers (I bought mine from a local hardware shop for £1.50) just doesn't have the oomph to melt stuff. If you press too hard it just slows down.
So as long as you have a light touch, it works really well.

Plus you could easily make different tips if you wanted too. I just use the little diamond coated ball they come with.