I think it depends on the use case. Activities allow for a lot more flexibility but they are not considered within the model because of the same reason. You actually get a warning now if you create a new activity in Pega 7 platform.

Typically, you are trying to transform the data as part of your flow. Thus data transforms can do the job really well. But data transforms cannot control when to retrieve data and when we save it either. While retrieval can still be done through configuration of data pages which source their data from an external source or the database, insertion of data is only available by activities.