This is an interesting question.
IMHO deleting images is an important part of DAM.
The issue is not necessarily about storage space as already alluded to.
The M in DAM stands for management and so it behooves us to actively manage our images rather than use Lightroom (or some other application) as a dumb receptacle for our images.
For what it is worth I delete images in several stages:
On import I will immediately delete images that are technical failures (hopelessly blurred, exposure issues beyond redemption, or those that are clearly compositional failures).
Over weeks to months I will review those images again and again. It will not be unusual for a particular image that did not initially stand out to eventually become the pick for that shoot. Sometimes I will be undecided about a particular composition for a long time but then suddenly realise that a subtle crop, or something, makes a radical difference. Sometimes images that I had hopes for I eventually have to abandon after years of wrestling with.
By the way I fully appreciate that others may have a much better tuned sense of the aesthetic and so may be able to make much quicker and more efficient decisions about the viability of an image, however, a bit of a break and a fresh appraisal always help.
Also, in my situation, having only engaged in "serious" photography for about seven years I find that going back through my image collection periodically allows me to delete early images rather easily because what seemed pretty acceptable four or five years ago is clearly substandard now. As of writing this I only have about 1% of the images I shot in 2007 still in my catalog, however, I still have about 65-70% of the images shot in 2013 in my catalog.
This question does have philosophical overtones and so some individuals will never delete any images and be able to advance sound reasons for their approach while some, like myself, progressively delete images and are able to justify that approach.
One possible solution for those who always have nagging doubts about deleting images despite acknowledging the need to manage their image collection is to keep a redundant archive of all the images they have ever shot while still actively managing their catalog by deleting images. If a mistake is ever made it would still be possible to re-introduce that image into the catalog from the archive. Since HD storage space is relatively cheap and dropping progressively in price this may be an option to have the best of both worlds.