[pabloem]

I don't think pretrained models are "beginner building blocks". They provide a kind of "base model" that can be fine tuned to your specific need. The advantage of not training the full model means you can save many resources (in design time and computation).

There are many boring but meaningful tasks in which this can be used. For example, I'm sure many industries could be benefit from image classification for very specific cases (e.g., fruit categorization). In those cases, you are not interested in the classification of "general" objects such as car, person, bike or horse (as provided by MobiletNet pretrained model), but you can use that model as base to classify different categories of fruit.

Therefore, you are right that they might not be very useful to build new ML algorithms or network architectures. But they are useful to build specific (and novel) uses of current neural networks.