Nothing worse than seeing "thanks, John" at the end of an email because it's a signature that's auto applied. And it stands out because the font/size/color don't match the body of the email
Not sure about other email clients, bit it is definitely possible in Thunderbird to set it up so the "Thanks/Regards/Best/whatever" is in the same font, style, etc. as the body and the rest of the sig is in a different font (that's my standard setup; compose it in TBird, save it out, extract, set as sig file, maybe tweak the HTML a bit).