Why would you go out of your way to disrespect someone?????
I'm a fan of nicknames but in general I find them really lazy. You're not calling a Nicholas "Nick" as a social surrender, you're doing it because you are a) as you stated, purposefully disregarding his given name or b) too lazy/busy/careless to pronounce the rest of it.
If I introduce myself to a stranger as "Madyson" and they try to refer to me as "Mady" I will ignore them. I will introduce myself (and any future children of mine) as the name I wish to be called and I think it's extremely rude and selfish for anyone to take it upon themselves to impose their own nicknames on anyone else.