I agree that, since TTF fonts are scalable, limiting font sizes is not necessary or desirable. There is a largest practical font size for any given screen size -- however "practical" may have a very low threshold if this is your only way to continue to read. Rotating to landscape increases the largest practical font size (since it is related to letters per line).
As an example, the Kindle only allows the selection of 6 font sizes but it actually fully supports 7 font sizes (at least for AZW/MOBI). It does not allow the user to select its largest size. This is perhaps to allow at least one size above the current default for headings etcetera, but I think it is a mistake on Amazon's part not to add it to the menu. See
Extra Large Print Kindle ebooks for screenshots of the Kindle's font sizes 6 and 7 (and the 8" iLiad's largest MOBI size). These Extra Large Print Kindle books are unfortunately no longer available from Amazon.