> Is the performance of manipulating a single direcory with 45,000
> entries even something we care about?

"We"?  I don't know.  *I* certainly do.

> Who would ever put 45,000 files into a single directory?

Anyone running netnews.

Anyone running a large maildir mailbox.

There are other less common examples I can cite if desired, but those
should suffice.

> Who would even put 100 files into a directory if getting a directory
> listing takes two or more screens just to display?

My home directory on my main home machine has 193 things in it: 108
plain files, 83 directories, and 2 symlinks.  Yes, the size of ls
output is mildly annoying, but so far not annoying enough to make me
clean it up.

I'd also point out that "100 files" does not necessarily imply "a
directory listing takes two or more screens just to display":

[Sparkle] 9> mkcd z
[Sparkle] 10> count from 1 to 100 | xargs touch
[Sparkle] 11> ls
1   12 16 2  23 27 30 34 38 41 45 49 52 56 6  63 67 70 74 78 81 85 89 92 96
10  13 17 20 24 28 31 35 39 42 46 5  53 57 60 64 68 71 75 79 82 86 9  93 97
100 14 18 21 25 29 32 36 4  43 47 50 54 58 61 65 69 72 76 8  83 87 90 94 98
11  15 19 22 26 3  33 37 40 44 48 51 55 59 62 66 7  73 77 80 84 88 91 95 99
[Sparkle] 12> 

Throw in two administrative files and that becomes just what a mailbox
with 100 messages in it looks like using my MUA.

/~\ The ASCII				der Mouse
\ / Ribbon Campaign
 X  Against HTML	       mouse@rodents.montreal.qc.ca
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B