60 FPS elitism is as annoying as 1080p elitism. Halo was fine at 30, even though i suspect that 30 was necessary to get a lot of stuff happening at once on past XBox-consoles.
Since tons of shit happens in Halo 5, its great that it runs at 60! I am not a fan of variable resolutions though, that may be because i play PC games mostly nowadays, but its acceptable as long as the drops are not too extreme.
As much as i can live with a variable resolution while playing on my TV, i can live with variations in framerates as long as they are not too extreme. So i wouldnt mind if they would stay at 1080 and let the framerate drop to the 50s when lots of stuff happens.
If its 343s choice to use a variable resolution, thats fine, even when i would prefer it to be different.
One thing they should not forget though is AF! Seriously, console games and their shitty texture IQ, its terrible.