On one of our company's King Airs you had to manually arm the altitude. In my eyes; compete bull!!!!. The other aircraft automatically arms altitiude hold mode
And with regards to capturing a glideslope above a preset altitude; GS capture inhibits ALT ARM, so this is not a problem. (However the altitude alerter systems continues to function and would give you a prelevel warning 1000' above and when going 300 feet below).