In ML, you were *prompted* after the import as to whether you wanted to delete the photos. That makes sense logically-- import first, *then* figure out if you want to delete. IIRC, you could even choose to delete selectively!
That was also risky. You were asked, when it looked like the photos had been imported, but so far you had only seen the thumbnails. Actually, the import was still going on in the background and you could not yet enlarge and check the photos, if the originals really had been imported safely without download errors.
We have seen many cases of lost photos because of deleting right from the Import panel. The typical error in Mt. Lion or any iPhoto version: You confirm to delete the photos after import, and the Mac starts to delete the photos from the camera/device right after the originals have been moved to a cache on the mac, before they appear in the events in the iPhoto Library. And with bad luck the Mac crashes, because one of the photos or videos cannot be processed. Or you imported more photos than you had free storage and the mac crashes. The result - all photos gone from the Mac, because the caches will be cleared, when the Mac restarts, and the photos will be gone from the card as well.
I would not risk that. I keep the photos on the card until I have seen all imported photos enlarged and have at least one backup of the new import. Then I erase the photos from the camera by reformatting the card, when the card is full.
When importing from the iPhone or iPad it will depend on if iCloud Photo Library is enabled or not. If iCloud Photo Library is active, there is no point in deleting from the Import panel, because all photos you are seeing are in iCloud Photo Library, and you can simply delete them by deleting them on any Mac that syncs with iCloud. The device mirrors iCloud, so you delete from iCloud rather than from the device.
If iCloud Photo Library is not active, then the "delete after Import" option will be available, but I would first import to Photos without deleting from the phone, then check the import and backup the photos, and then use Image Capture to delete them from the device.