Certainly secretarials are common and they always have been. I think most people who collect for any length of time know that. A look in the back of Autograph Collector Magazine now will give you addresses of mostly celebrities that the average person hasn't heard of. Ever heard of Nora Zehetner and Ruben Dario? I haven't,.
The experienced collector knows that busy celebrities who get deluged with mail isn't going to have time to answer mail even if he wanted to. But it is a misconception that celebrities don't have life cycles just like the rest of us. I have corresponded with Lloyd Bucher (he was the Commander of the USS Pueblo that was captured by the North Koreans) and George Wallace (I have a letter he wrote me talking about a phone conversation with JFK Jr.) I wrote the imprisoned Panamanian dictator Manuel Noriego in prison a couple times and you letters back both times, pleading with me to support him. Those autographs interest me more than established movie stars. I do collect some of big name entertainers, but I get them mostly on letters, contracts, cancelled checks etc.
One thing that is overlooked by a lot of naysayers is that people collect for two reasons - fun and profit. If one is doing it purely for fun, any response is welcome. It wasn't until the money angle crept in that people even thought of, much less discussed, whether that Marilyn Monroe autograph was actually signed by her. If it came back from the same address that it was sent to the collector was happy. There are some people who still feel that way and more power to them. Happiness is a good thing.
Then there are those who collect for fun and profit. I will guarantee you beyond a shadow of a doubt that those who educate themselves and "selectively" send autograph requests though the mail will be rewarded in the future. When it comes down to selling time, nobody's going to separate the in-persons from the "through the mails" based on anybody's word. It's either real or it isn't - and a lot of those autographs that were gathered while the celebrity was walking or getting in a taxi are going to look like this *!7&&%# more than a legit signature and many of those signatures that came back in the mail are going to look real, and a lot of them are going to be real.
Being an educated collector comes down to a lot more than "if I didn't get in in person, etc." The only one that could verify that it was in-person was the one who got it and if those were the conditions for authenticity, it would be worthless to everybody else.
If I send an astronaut his autobiography to sign, he signs it, I put it on eBay and double or triple my money, who's losing - the astronaut, me, the guy who looked at the autograph and made his mind up that the autograph was legit and decided to buy it, or the person who decided that getting stuff signed through the mail was for losers?
Fun and profit is a good thing. Btw, I would only sell a signed book by an astronaut if I had an extra copy of it - at this stage of my life anyway.