(1) Malzahar is NOT just his ultimate. His ultimate is easily his strongest ability, dealing lots of damage and suppressing but he has 3 other abilities. Picking up a Rylai's Scepter helps too. With high ability power and some cdr/mana regen (like blue buff) Malzahar can silence and slow multiple enemies with his Q (Call of the Void) or damage and slow with his E (Malefic Visions). Also, I highly doubt everyone on the enemy team is going to be buying Quick Silver Sashes. However, if they do, you have made them spend 1640 gold (to a carry this is a BF Sword) on an item that only really cancels 1 of your 4 abilities. As mages go (AP carries) Malzahar is particularly strong late game since he has an ability that does % max health damage. This lets him take out some tanks on occassion, not that you should be focusing them, but when the opportunity to pick up a kill appears why not...
(2) Voidlings got updated recently. They used to cause a few problems for Malzahar on occassion including: stand in the open when you are trying to hide in a bush, attacking enemies you didn't need them to attack etc. The update significantly improved the voidling AI as far as I am concerned and removed most of the annoying things that made a voidling a potential nuisance for Malzahar himself. Voidlings have also saved me now and then by appearing when a skillshot is inbound (hitting the voidling instead of hitting me).
I would say that Malzahar's "weakness" to this one item is not sufficient to me not play him. Every champion in the game has some weakness, some carries cant punch through a Thornmail, some mages have a really difficult time getting through a little magic resist but it isn't these small things that make a champion good or bad, playable or unplayable. Malzahar is a strong AP carry and you can get around a few enemies picking up a Quick Silver Sash by just playing smart and using your other 3 abilities to their fullest. Also, you are making the spend 1640 gold, which shouldn't be underestimated, just to counter one of your abilities. This means they didn't buy something else (damage, armor, attackspeed, etc.) which they would have preferred to buy, had you not been ruining their day as Malzahar.