Not harm in general (a matter of taste I think), but for example there would be quite a few characters to employ who do not so widely differ from the LBT characters. Whenever there are new characters in LBT they either differ from the characters very much (up to the point of villain characters), or they are in need of help. With the group of Dink included into an LBT story one would need something really to tell. Just letting the two groups do what they are doing anyway all the time and which both groups could do all by themselves one would need something that made sure the characters were there for a purpose other than just having a long list of characters. Some LBT movies have been criticized for neglecting Ducky, Petrie, and Spike on behalf of Littlefoot and Cera. With Dink, Amber, Scrat, Flapper, and Shila added to the group there would be twice as many characters to share the action and the result might be that many of the characters might be mentioned but left with little individual action.
No doubt there are interesting possibilities (e.g. it would be funny to see almost Cera-like Flapper with the much shyer Petrie or Cera who would not be accustomed to that kind of attitude from a flyer), but with ten main characters one really needs a lot of stuff to do and say lest some of them be nominally present only.