Then change it to 'Sent from my phone.' - changing it is a couple clicks, and anybody advertising a product in every message they send deserves any derision they get.
Derision? Do you also deride people wearing shirts with a crocodile on them? People driving cars with a little lightning bolt in a circle? Businesses issuing paper invoices with the supplier's name printed on the invoice form? (Practices may vary by country.)
Leaving this one line unchanged says ~= zero about a person.
Knowing which device they are on helps me in small ways I don't even consciously remember. For example if someone replied and I knew they are on an iPad this moment, I know that I probably can't expect them to easily sign the PDF agreement I'm about to send and if I really want it. Ack urgently, I should provide an iPad-friendly method to sign and return.
This becomes crucial when you are doing back and forth between potential customers.
If knowing the user agent is seriously crucial for you, you shouldn't rely on "Sent from <user agent>" messages that can be disabled. Instead, you should use an email client that provides functionality similar to the Display Mail User Agent extension for Thunderbird:
Leaving this one line unchanged says ~= zero about a person.