Exactly what came to my mind as well. A lack of care when you write can signal that you do not know how to write well, or it can signal that you don't care to write well. Neither of these are signals I want to send, both harm my reputation.
It can also be a signal that you're too busy to write well in that context.
There are two elements to writing well. One is the efficient communication of content. The other is communication of social register, relative status, and power relationships.
Hitting the right social register is an impedance matching problem that depends on the target audience.
If you go too high you risk sounding snobby, pretentious, and condescending. If you go too low you won't be credible.
People often assume this means that if you lard your verbiage with orotund circumlocutions and vague implications you'll be hitting those high register top notes.
But in fact social register doesn't map neatly to grammar/reading age/vocab.
SMSspeak in an email can - ironically - hit too high a social register because you're implying you don't have time to write a more detailed and conversational response, and you're not making the conversation a priority.
This is orthogonal to any actionable message content.
