star wars - Why was it a mistake to come out of hyperspace close to Hoth?

From the script:

Alarms sound throughout the hidden Rebel base.  In the control room, a 
controller urgently gestures for General Rieekan to check a computer 

scan.

            CONTROLLER
    General, there's a fleet of Star 
    Destroyers coming out of hyperspace 
    in sector four.

            RIEEKAN
    Reroute all power to the energy 
    shield.  We've got to hold them till 

    all transports are away.  Prepare 
    for ground assault.

Rieekan exits hurriedly.


[...]


            VADER

    What is it, General?

            VEERS
    My lord, the fleet has moves out 
    of light-speed.  Com-Scan has 
    detected an energy field protecting 
    an area around the sixth planet of 
    the Hoth system.  The field is 
    strong enough to deflect any 
    bombardment.


            VADER
        (angrily)
    The Rebels are alerted to our 
    presence.  Admiral Ozzel came out 
    of light-speed too close to the 
    system.

            VEERS
    He felt surprise was wiser...


            VADER
    He is as clumsy as he is stupid.
    General, prepare your troops for a 
    surface attack.

            VEERS
    Yes, my lord.

Why exactly was this a bad idea, and what was Vader's preferred strategy? The Rebels can apparently raise their shield instantly, so it seems like they would do that anyway, surprised or no. In fact, since being surprised gives them less time to prepare, it seems like this is a good strategy on the Empire's part.

Is the issue that the Imperial forces didn't have enough time to prepare, and it would have been better to come in farther out and spread out, as this answer says? If so, why wouldn't forming up while already in the system also give the Rebels warning, and give them even more time to escape? All I can really think of is that they would have somehow known the range of the Rebel sensors, dropped out of hyperspace just past them, formed up, then quickly flown into range, but that seems hard to infer from the exchange between Vader and Veers.

Answer

Darth Vader's strategy was to exit hyperspace on the perimeter of the Hoth system and drift into Hoth orbit undetected. From the Wookiepedia article on The Battle of Hoth:

Vader and many of his top officers had planned a sneak attack, jumping out of hyperspace at some distance from the planet and taking advantage of the fact that the meteor activity made it difficult for the Rebels to spot approaching ships. They would then initiate limited long-range orbital bombardment to destroy the base, along with any spacecraft and defenses. Major General Maximilian Veers would then mount a sweeping ground assault to capture any survivors.

Admiral Ozzel foiled this plan by jumping in too close - from the Wookiepedia article on Hyperspace:

An interesting phenomenon associated with hyperspace travel was Cronau radiation. This was a short, but powerful burst of radiation which was generated when a ship entered and left hyperspace. It could be detected by properly aligned sensors from a few light-seconds away, often well outside normal sensor radius. This was how the Rebel base on Hoth was able to prepare for the oncoming attack when Admiral Ozzel mistakenly took the Executor and its battle fleet out of hyperspace too close to the system, rather than approaching stealthily from outside the system.

Conceivably, a single missile disguised as a meteor could have taken out the Rebel base's power or shield generator before the shield could be raised (witness how the Imperial probe droid approached Hoth undetected).

Now whether "Cronau radiation" was canon when The Empire Strikes Back was written, I don't know. I think the main idea in the film was that Rebel sensors could detect big blips in Hoth orbit, like a massed fleet of big ships appearing out of nowhere at great speed, but cannot detect a smaller vessel like the Imperial probe droid.

It's also implied that maintaining the planetary shield around the Rebel base is both costly in energy consumption and could compromise the secrecy of the base by emitting a big radiation signature. Hence the Rebels don't simply keep the shield up permanently.