Starfish - the swim is the shortest portion by time and therefore does not gain you a lot in terms of overall placing unless you are able to hold a decent time in the bike and the run. The bike is the longest portion by time, so there is a lot to be gained by being a good cyclist. The most important factor for the swim is effort level. If you swim well and finish decently, then you have a much better chance of biking well because you're not totally thrashed from the swim. There's a little saying that goes like this:

You can't win a triathlon in the swim, but you can sure as heck lose one.

Or something like that.

Numbers on an Oly:

Jane swims 2:00/100 = 30:00
Bikes 30 kph = 1:20:00
Runs 5 min km = 50:00

If she improves by 10% on each event what does she gain?

Swim = 1:48/100 = 27:00
Bike = 33 kph = 1:13:00 ish
Run = 4:30 min km = 45:00

So you gain 3 min on the swim, 7 min on the bike and 5 min on the run for the same percentage of improvement. And you might know from experience that gaining 10% in the pool is a lot harder than gaining 10% on the bike.