^^^ False. That's just a stereotype. Any jaded, lazy, or un-knowledgeable sound guy will be ticked at you.
Yes most soundguys only take a DI from the bass instead of the head, but they have no clue why. It's one of those annoying 'traditions' that gets passed down without giving a reason, and young engineers just do it because 'that's the way you're supposed to do it'. So IMHO its the bad sound guys that do this, because they don't know any better. Furthermore, once they get that 'clean signal' they have no clue what to do with it, and butcher your tone, or completely buries it.
I think the rational is that an engineer knows their DI works, but they don't know if your amp is junk or not. So they don't take the risk. However, with just about every modern bass amp providing a built in DI, most of which sound fine, have a ground/lift switch, a -db pad, and have a pre/post switch if needed, there's really no good reason not to try it.
What I do is if the bassist clearly has nice gear, or an amp I'm familiar with, I take the DI from there head. And if they have an obvious attention to tone, I'll gladly use the post send. I leave a DI box sitting on top of their amp as a back up. If anything goes catastrophic, it takes .5 seconds to switch the connections.
Any good sound guy will know there's no one magic bullet for any situation. There's always variables. Take this situation for example, what if a kid comes in with a squire p-bass, but has a nice Ampeg head (say a svt-3pro). They probably get a decent sound through the amp and its onboard DI, but just DI'ing the bass sounds like, well, a squire.
Or how about any bassist that uses effects pedals? trust me, pedals sound different (and usually bad) if you go directly into a PA, then if they're running into the impedance of the amps input, and/or interacting with its preamp.
Anyway, to close the rant up. There were a lot of threads here about how soundguys take a straight DI, and refuse to mic a cab or take a post signal out, and how much that pisses off most bassists. (these threads all go down a bad road and were banned). I just want to say that as a soundguy, I hate that most soundguys do that.
p.s. this is a general rant derived from boredom, not directed at MNairhead or anyone else.