It'll be less important now than it was 3-6 years ago, but get ready for all those FeedBurner subscriber count widgets to show huge subscriber # crashes as Reader no longer checks in.
Back in the day, they were a sort of informal auditing system and definitely helped me land advertisers for my blog (simply because I could "prove" I had 20,000 readers or whatever).
Thankfully I ditched the Web and moved to e-mail and know exactly how many subscribers I had, but this was certainly more luck and not any great piece of foresight on my part ;-)
This is going to sound antagonistic, but I'm genuinely curious: How many subscribers did you lose when you switched from RSS to email, and how do you know that all of them are receiving and reading your dispatches?
Personally, getting an e-mail instead of an RSS feed is sub-optimal - I can't easily add things to pocket from e-mail. I'm also wondering how many people trained their clients to re-route to spam.
I didn't do a like for like switch from the Web to e-mail, it was more specific for the format, so I can't make a direct comparison sadly. Because e-mail proved way more successful for me, I've basically let the Web properties die but that was not the original intent.
With e-mail, the engagement and revenue are way higher, and it's possible to get an accurate feel for how big the audience is (as in, I know I have 123k subscribers or whatever) and how many of those are engaged, reading, and clicking on stuff.
I was very skeptical at the start and just tried it out because of all the e-mail newsletter hoopla on HN 3-4 years ago but it's now my main business and growing more rapidly than my Web publications ever did.
There are, of course, many who are not fans of e-mail but like Facebook oriented businesses can make good money even though not everyone likes Facebook.. so too can e-mail companies do very well off the majority who are still using e-mail.
Back in the day, they were a sort of informal auditing system and definitely helped me land advertisers for my blog (simply because I could "prove" I had 20,000 readers or whatever).
Thankfully I ditched the Web and moved to e-mail and know exactly how many subscribers I had, but this was certainly more luck and not any great piece of foresight on my part ;-)