However, each time running another version it needs reindexing because the datadir
contains only the different wallets but all other files are the same files for every wallet.dat.
AFAIK, the client does not reindexing the blockchain.
It searches in already downloaded blocks transactions related to the current wallet.dat
multiple instances at the same time would likely cause locking issues with the block data...
...only if programmers are dumb
there is no significant problems to handle several wallet.dat databases for one blockchain
of course, it is difficult to rewrite current ass-written code.
and i do not see big reasons to do it