It's just a frustrating and misleading stat for Riot to display. One ADC can have a 700 DPM compared to an ADC with 500 DPM simply because the first ADC doesn't close out games until 5/6 items every game, whereas the 2nd closes the game out earlier.
I would agree that damage to champions is not proper evidence to contribution, as (an example) Singed's whole thing is entirely focused about creeps. However overall damage, and by contrast, %DPG, would always show that player's relevancy.
Note: As a support player myself, we should all understand that cc and zoning do not show up in this, but they don't show up as proper evidence towards a contribution already with the DPM stat.
What Riot could do that might really help is developing stats that are role-focused, similar to actual sports. Football? QB stats are passes, completion %/rate (note, %), yards gained. Defense stats show sacks, avg yards per game by opposing teams, interceptions. RB's stats yards per carry, TD's, etc etc.
For the World's scene, I would find it very impressive if they managed stats for Mid, ADC and Top for %DPG, Support stats could show number of wards placed and removed per minute (per minute works in this sense, as this will average out and a % method is pretty unrealistic to implement), Junglers could do kills/assists per gank.
These are merely examples towards what I'm trying to suggest. As I see it, if Riot wants to solidify the notion of Esports to be treated as a sport, (1) they should aim to provide the most relevant stats they can, where %DPG will always be a better visual of contribution than DPM would ever be, and (2) that Riot can present League as a real sport by designating proper stats to the proper roles/positions like football, soccer, baseball, and any other sport does (for Worlds, they wouldn't need to seek it at the unprofessional level).