Mercury is a hard place to get to - particularly if you want to stay.

An Atlas V 551 would probably get you there in a lot less time, but you would also need a much MUCH larger spacecraft with a much much larger fuel tank to to the orbital insertion manouver. i.e you accelerate an extra 5km/s here, you'd need to drop all that speed again when you get there.

What Messenger did by using a Delta 2 is two thing

1) Save >$140M over the price of an Atlas V 551 (i.e. Messenger could not fit the Discovery program budget with such a vehicle)

2) Reduce the speed at which it arrives at Messenger, thus requiring a much smaller delta-V to do the orbit insertion.

It's using a lot of planetary flybys over the next few years, which in themselves will offer some scientific value, if not just for calibration purposes. By using those flybys it can 'nudge' its way closer and closer to the orbit of Mercury.

New Horizons is an odd-ball in current planetary exploration, a flyby. It doesnt need to stop. Much of the reason it can be launched so quickly is that it is so light - less than half the weight of Messenger - and indeed, if you take all the fuel out of messenger, the fuel it will use for orbit inserstion - it weighs about as much as NH.

Doug