Yo rbwinterlink and Fred:
It's the storage space that's issue for those 15IPS tapes. I used 3600 foot reels, which are 10 1/2 inches. My 15 IPS is a TEAC 3300. I have maybe 50 of those reels, and maybe 120 7 inch 7.5 IPS reels. A 3600 foot reel of 1 mil tape is good for about 45-50 minutes at 15 IPS, so you can typically get 2 sides of an LP on a 10 1/2 inch reel. A boxed 10 1/2 inch reel takes the space of about 3 LP's.
For me, back in the day, the big advantage to the 15 IPS speed was that it gave great flexibility in editing. You could cut the pops and the clicks right out of the records with a razor blade. You could also edit program material, either musical material, or sound effects, or spoken word. I produced a tape of a speech, a closing argument from a trial, long time ago for a company that sold them. I was required to edit the material to smooth out the subject's delivery and get rid of the stray sounds and coughs. It's quite easy at 15 IPS. At 15 IPS, a typical pop is about 1/4 inch long. There is an issue if you have a big bunch of pops or clicks. It's that the record actually gets a little shorter once you cut them all out. That was one of the issues with transferring 78's to tape and thence to LP.
For me, the editing flexibility on 1/4 inch tape is overcome by the ability to use click remover software, and the additional flexibility of the editing software that is available for digital editing. Also, the storage space involved is much less for digital files, even if you are using WAV files. As an apartment dweller, the storage space is important to me.
There is nothing cooler than watching those 10 1/2 inch reels going around. It realy is impressive, and when you rewind, make sure the reel locks are tight...
And good luck from that cramped old guy, who wishes he had the space for a couple of hundred 10 1/2 inch reels,