Not necessarily true. Most jetted tubs are available as drop-in without a tiling flange. Many of the manufacturers offer an add-on tiling flange. As long as you have one of these (one with a factory tiling flange is preferable), a shower is possible to produce that will be water-tight.
Now, considering the recommended monthly maintenance of a jetted tub, you may not want to use it daily for a shower...keeping it clean around the jets when used in that manner could be an issue, but doable.