"Real deadlifts?" Really? Worry about yourself, someone else pulling sumo doesn't effect you at all and definetely shouldn't piss you off.
Anyway, my opinion is that most of the people that pull sumo aren't even built for the style or are just doing so to get more out of their gear. Sumo is better for guys with longer torsos and shorter limbs (basically, guys that are naturally better at squatting and benching and struggle more with conventional deadlifting) who probably won't ever be setting records in the deadlift solely because of their frame.
In my experience and from what I've read though, even if you plan to pull in a meet or for your heaviest attempts, you should still pull conventional a lot of the time in training. The consensus seems to be that conventional builds back strength and the whole posterior chain better than sumo does. So, if you were to only pull sumo in training or to focus on it more, your back would probably weaken as a result.
I'm built perfectly for sumo, and I pull conventional about 2/3 of the time and I've noticed that my sumo pull gets MUCH stronger when I go back to it, in comparison to when I'm pulling sumo the majority of the time. I even read a post on wannabebig where Dave Hoff said he pulls conventional almost exclusively, and only pulls sumo for his speed deadlifts to keep his sumo technique/strength up to par, and he always pulls sumo at meets and seems to be doing just fine.