A little like comparing apples and oranges both are sprinting but very different or pretty different situations.
You can kind of answer this yourself, find a flat (quiet) piece of road put your bike in a gear you can reasonable get going from a very slow pace and leave it in that gear. Now ride from a standing start, up to a reasonable pace THEN accelerate to max, you will very soon find it hard to stand up as you begin to spin out. Sit down and you will still be able to put the power down.
Now do the same thing but riding onto the same stretch of road at a fast rate, begin to up your pace, this time allowing yourself to use the gears you will be able to get into a gear that you can still sprint standing up.
I disagree with the comment further up the thread about standing up being less efficient, well it's true to an extent but standing up for a short period of time to sprint allows you to use your whole body, standing is less efficient for long periods of time, but for a short period of time you need to get everything out thus standing is efficient! Albeit less areo but Cav is still pretty compact when he sprints and keeps low.
If it was less efficient I kinda of think that the road boy's might not do it!?!?! I think both do what's best suited for their event. They are pro's they don't need to search the internet for answers! (I don't at all mean that the OP's question is not a good one, it is, it's the replies that seem to suggest that Cav is doing it wrong that are funny!)
It's a huge amount to do with equipment track riders need to be able to accelerate very quickly and get on top of the gear, they don't have the luxury of gears they have to choose something that's small enough to enable them to get up to speed but still big enough to hold the speed. Hoy's sprint is a very different kettle of fish to Cavendish's sprint, track sprints are from a very slow pace up to a very fast pace, where as a road sprint comes off riding 100's of miles and being delivered to the line at a pace that's much higher.
Not sure if I have explained that well or not!
Re would track riders be quicker with gears, hard to say, some ways yes some ways no but their sprints are so short (generally) I doubt it would make that much difference...and will never happen so a moot point really!