The problem is that you don't care about email clients as distributed by senders, you care about the distribution of recipients. Especially if you're a marketer trying to gauge what kind of HTML you can use in your email.
The only people who can really use the headers to identify which email clients are in use are people who maintain very large ISPs. Although, even then, keeping track of the results of the IMAP ID command are probably more useful.
The only people who can really use the headers to identify which email clients are in use are people who maintain very large ISPs. Although, even then, keeping track of the results of the IMAP ID command are probably more useful.