Alright. The changlings feed on love right? Then why in the name of Celestia would Chrysalis just toss away an Alicorn that can mass produce the stuff at will? When she's taunting Twilight in the caves under Canterlotte it's pretty clear that she intended to have Twilight do away with the real Cadence so my question remains. Why throw away an unlimited source of nourishment?

Bam, right there! You see we're never really told why the shield is up in the first place. Sure, there was a threat made on the city but by who? We're lead to assume that it was from Chrysalis. But Chrysalis is already inside the shield!

It's possible that the reason Chrysalis had to impersonate Cadence was because the shield was lifted in the first place, meaning it was not her first plan, but rather her way of adapting to changing conditions, honestly that would make her more awesome.

But who knows, maybe I'm overthinking things. :lol

Cadance's magic is completely at will. Why would she help Chrysalis?
You have to remember that Chrysalis is invading Canterlot. She had no intention of making friends. She had an army and she was ready to conquer. Cadance wouldn't use her spell to feed Chrysalis or the Changelings since they were enemies even before she had trapped her in the mines.
She had to do away with the real one so that Shining Armor, who was under Chrysalis's mind control spell (Which was made clear that took effect over time and wasn't immediate) wouldn't be able to encounter the real Cadance and ruin her plans.

So, her plan was:
1. Trap the real Cadance in the mines
2. Impersonate her
3. Begin her mind control spell on Shining
4. Use Shining's love for her as a source of food
5. Put Canterlot in a state of alert by making a threat (Doing this would divert attention from her, making her obvious differences from the real Cadance be unnoticed because everyone is too busy worrying about the possible attack on Canterlot)
6. Imprison Twilight and have her axe off the real Cadance
7. Once Shining is completely under her control, she'd have control of the Royal Guard
8. Invade Canterlot
9. Profit

She pretty much had a solid plan, since it's been confirmed that the Changelings can feed of love alone, the love doesn't have to be directed towards them (So the love Shining feels towards Cadance could feed other changelings than just Chrysalis).

So, if I'm reading this correctly, Chrysalis sneaked into Canterlotte to remove a shield that wouldn't have been a problem if she hadn't sneaked into Canterlotte?
Though if the shield was already up like you say then why doesn't her army just sneak in the way she did?

I guess we are over thinking this. :lol

You guys are over thinking it. Her plan was crystal clear. :p

O_O Wow... Well, I must say I stand corrected. Well played sir, well played. :lol

I meant it more like the threat being her way of testing Canterlot's defenses, once she realized they had access to a frigging force shield she thought up the plan with Cadence.

Overthinking MLP episodes, it's a way of living. :lol

Chrysalis had a solid plan. Additionally, by creating the alert on Canterlot, she would have Shining make the shield around the city. This meant that ponies wouldn't be constantly coming into the city to possibly alter her plans.

I guess her ultimate undoing was letting the mane six into the city, more specifically Twilight. :lol