They do have inherant resistance, but this depends on drum size and is not something you can alter, of course you can alter resistance by changing gear but the max resistance is often not enough to achieve the training goals of many cyclists. Except in the case of the hybrid systems with resistance units. If you are thinking along the lines of HIIT, a turbo is probably the best bet. , Also at some of the higher intensity level's the stability of a turbo really is something of a virtue! There have been times when dismounting my turbo at the end I am so knackered, I am sure if I was using rollers I would have fell over.