The Flight Director will always give valid pitch cues (speed control by elevator) when changing level, no matter what thrust setting is used - thus making your life a little easier.
Are you sure? I've seen many a cadet in the sim select a change of MCP altitude but not set an appropriate thrust. As you say the FD is in Speed mode for pitch and if there is no thrust increase in level flight it will stay level to maintain the speed. Or am I missing your true meaning?