you can but Apple decides when you can actually use it and when you can't and have to use the Apple provided keyboard

Note that there are very good security reasons for this, as the keyboards can read everything you type. There are contexts in which defense in depth is more important than convenience.

I understand the security issues. I still feel I should be able to choose who I trust. I trust my keyboard maker more than I trust Apple. Further, my keyboard has my completions, my autocorrect history, and I'm used to it. Every time Apple's crappy keyboard appears it's as bad as someone asking me to type an a Dvorak keyboard.