Don't use ksh then ... just use the POSIX sh !
If you don't want to use new things because may not be compliant with old one, then just stick with old one
No, seriously, if you are achiving script that are intended to perform tasks with high compliance constraints for migration , or because used on a very heterogeneous environment or whatever, of course you'd better stick with things that are the most portative possible.
Otherwise, i would go for taking advantage of the functionnalities offered by the shell you are using.
By the way, i think that ksh is already very portative.
The most important is to have a code
1. resource friendly
2. easy to read/maintain so you should comment what you do so someone else can read it easily