Bfisk - you are right, it ought to be expected from a computer system to lockout the alt capture mode if on a glideslope. After all, it should not be more than one line in the program code. But the old(ish) DH8-300 is unable to do this on its own and relies on manual selection of that "ALT SEL" button, so as the -400 had to be certified to be flown on a common rating, it was kept that way.