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:
i suppose this opinion would vary by recipient. more importantly, already addressed, is the context it provides around the time spent on a response. in most cases, when responding on a mobile device, you simply can't put the same time into a response. i think it's considerate.
I agree. I change it to say "Sent from a mobile device". There are cases where a message mandates a reply but a mobile device isn't to an ideal platform. iPhone only recently allowed file attachments beyond 1, and that was if you remembered to attach it first. I don't think you could even attach to replies.
Many emails do necessitate a reply but you aren't going to be able to spellcheck and put the time into it as you would on a desktop. Afterall, you are mobile.
Stating so in the signature lets the recipient know you are giving them your full attention as much as the technology and situation will allow. I've always felt it was a good thing when users noted this.
And of course, as someone has mentioned, it is the default on iOS devices. Then again, users unable to edit their default signature probably aren't giving you their full attention and the reply will amount to yes/no or worse, not answer a single question with any sort of detail. Not that if the user were on a desktop that would change a bit. I'm not sure how these people are employed as I would fire so many people over the way they handle email.