Like others have said, the no-save part is huge.
We recently finished up Paizo's Shackled City campaign. Not to spoil much, but the BBEG had enormous saves - and we knew it going in. So my mage was packing exclusively no-save spells, or spells where the secondary effect was good enough. (Side note - did you realize that an Empowered Orb of Cold is pretty much completely better than Polar Ray?)
Ray of Light managed to keep the beastie blinded long enough for us to both pile on the damage and regroup.
So as your only 6th level spell - sure, go Disintegrate. But as you get up there in levels and the saves start scaling, it's worth it to keep your mid-level spells around as no-save attack spells. Also, lack of save = scroll bait.
We recently finished up Paizo's Shackled City campaign. Not to spoil much, but the BBEG had enormous saves - and we knew it going in. So my mage was packing exclusively no-save spells, or spells where the secondary effect was good enough. (Side note - did you realize that an Empowered Orb of Cold is pretty much completely better than Polar Ray?)
Ray of Light managed to keep the beastie blinded long enough for us to both pile on the damage and regroup.
So as your only 6th level spell - sure, go Disintegrate. But as you get up there in levels and the saves start scaling, it's worth it to keep your mid-level spells around as no-save attack spells. Also, lack of save = scroll bait.