I know in my situation I was thinking about 3 things:
1. I use my tools and need them to be there for me and if I should break one it needs to be replaced. Since my Driver couldn't guarantee that anymore on the BS sockets, I wanted to find a set that had backups readily available. I don't want to have to search eBay should something happen.
2. Those sockets have been in my box and in use since 1988. I purchased them used from a friend and mentor who exposed me to working on and riding old British bikes. They were his spare set that he sold to me for a song because I was a young kid with limited funds. They now have sentimental value and I would like to keep the set complete. Some of the sockets are showing their age, 1/4BS and 5/16 BS mostly, and would hate to get them to a point where they are just worn out.
3. The Koken replacements are 6pt, well made, and relatively inexpensive. I think I paid less than $100 for the new set and they should last me at least another 25 years.
So, that was my rational. I don't have many tools that fall into the "keeping them for sentimental value category" but the BS 12pt sockets and my old KR toolboxes defiantly fall into that category. Even though the Koken set replaced the old SO and the new KRL replaced the old KR both the old ones aren't going anywhere they just get to take it easy from now on.
As with everything under the sun, others may/will have much different ideas on the matter but those are mine. In fact, in typing this response I've spent way more time thinking about it than I did when I decided to buy the replacements.
Cheers,
Chris