There is little doubt that there are benefits to their current model. tvanantwerp covered some of those benefits.
But there are costs and benefits that don't appear to be included in their current calculations. If anybody doubted the difference that lots of little contributions can make need only look at successful crowd-sourcing systems and campaigns.
I believe that most non-profits have been told to go after the big fish donators and if they have to destroy all the little fish in the process, "oh, well". My point, and based on the other comments, I'm not alone, is that the little fishes' contributions may add up and be worth more than the big fishes with a lower overall cost (because it costs to produce and distribute those tons of mail).
Out of that group, donors are checked against databases of known giving history and net worth. These databases aren't always accurate or comprehensive, but it helps fundraisers to ID people who might be able to give more--sometimes a lot more.
When high-value prospects are IDed, they usually get assigned to a major gift officer who starts building a personal relationship to solicit very large gifts. This is where the charity makes the majority of its revenue. I usually see 95% of revenue come from the top 5% of donors, largely individuals.
If you're uninterested in receiving lots of mail, most charities will try to accommodate that request. However, resources are limited and dealing with the problems of $XX level donors often takes second place to the problems of $XXX,XXX level donors. I've seen stacks of returned direct mail several feet high from people who moved, died, wanted no more mailings, you name it. It was always on the to-do list to remove those names, but never at the top of the to-do list.
"databases of known giving history", "databases of net worth"...
In particular, I contacted them asking if contributors were put on mailing lists. I was told that regular membership contributors were added to several mailing lists, but that one-off contributors were only sent a small amount of ACLU generated mail, and if I put a note on my donation to the effect of "do not send ACLU mail" then I wouldn't even get that much. I set up my bank's auto-bill system to send them a $100 monthly check (i.e. regular "one-off" contributions) and had "DO NOT SEND ACLU MAIL" in three different locations on the check and that phrase was the only thing in all caps on the check. Within two weeks of the check being sent I had started getting non-ACLU mail that had a typo from that check. I immediately cancelled my bank's auto-bill to them. That was more than 3 years ago. So their spam has cost them at least $3,600 from me alone.
I don't think this is one of their priorities.