The devices with ldpi / mdpi are essentially low profile devices which have severely constrained resources like memory. If you are using large images for such devices, the issues you may face are:
- An app that is slow to execute as large images have to be decompressed by the relatively less powerful CPUs
- OutOfMemory errors due to the images not fitting the heap size of these devices.
Edit: This answer assumes that the OP understands the reason for Providing Alternate Bitmaps and is only interested in knowing the risks in not following the Best Practices.