Actually, a FreeBSD or NetBSD rc system does indeed care, for that very reason. rc.d scripts are sourced by a /bin/sh shell, using the . command. See run_rc_script on the rc.subr(8) manual page for details. Ironically, it's daemontools and daemontools-like systems where what you say holds the most widely. Tools such as execline can be and are in practice used in "run" programs.
It doesn't look like OpenBSD has such a requirement (the start_daemon() routine in /etc/rc just calls each configured daemon's initscript as-is from /etc/rc.d without specifying sh or ksh to execute with); I had (incorrectly) assumed that this was a general BSDism rather than an OpenBSDism.
