I think it's a sure bet that A+ credits are "safe", as in:
--They won't suddenly expire or disappear
--At some point in the future where there is no longer an A+ prorgram, some sort of means will allow unused A+ credit to move into the Southwest world
In that way, it's almost certain that credits are "safe".
However I don't think most people are concerned about that. They have different issues which cannot be answered:
--When I want to use my credits, will there be any AirTran left in the markets I want to use?
--When there is eventually some sort of means to roll A+ credit into the Southwest, will the transition be reasonably equitable and useful? Or will it be an arduous, strings-attached process?
The first issue is already looming. Seattle and Des Moines residents with A+ credits won't be able to use them later this year as Southwest takes over. More cities may well follow soon. Here in Milwaukee, just about everything headed west which AirTran serves now will be 100% Southwest by fall. And if rumors elsewhere are accurate, MKE and BWI will be all Southwest soon except for flights to the ATL hub. If/when that happens, there will be countless "safe" A+ credits which won't be very useful at all.
As for how A+ credits will some day roll into Southwest, who knows. It could be something as brittle as on MM/DD/YY the program ends, they mail out a voucher for one-way Southwest travel for every ten A+ credits in your account (remainders ignored), the credit expires within 1 year, and you have to jump through hoops to redeem. Hopefully it's not that unfriendly, but it certainly could end in a way like that without anyone losing their credits.
Maybe they have something up their sleeve for cross airline earning or redemption which will come well before the ellusive code sharing now promised for sometime in 2013. But this sort of uncertainty is very real, and sending out an e-mail saying "your credits are safe" does nothing to ease it.