Counts As Armies

So one of the things I'm hearing about are people that play one space marine army say Black Templars, for example, then when a hot new codex comes out they want to play their army with rules from the new codex. This irritates the hell outta me. Luckily I haven't had to deal with it yet. Why would someone play an army that is out with its own rules then just because a new army codex comes out they want to use the new rules? Now I'm not talkin about those people that put a new army together with a cool idea and doing counts as. Like a genestealer cult that uses the nid codex rules and does counts as for different models. I have no problem with someone doing a cool idea for a new army and using the rules from an existing codex... new or old. That's how you get cool things like Dave Taylor's Blood Pact army with the cool trucks and tanks. He creates a model and then uses the rules for an existing one. Like a truck with a big tank and heavy flamer and using it as a hellhound.

But why would you take an existing army like Black Templars or Blood Angels or whatever and play them with Space Wolf rules? Just because the Space Wolves are the new flavor and have some cool stuff? Lack of imagination? Comment back and give me your take on this.