Why electrum server instead of direct bitcoind jsonrpc?

[josvazg]

I am relatively new to bitcoin and electrum.

One thing I don't get about electrum is why does it need to have a non standard server?

Couldn't the client connect directly to bicoind servers nodes using jsonrpc?

And about security, couldn't the client connect to multiple servers in parallel and ask for things in parallel (sort of a map reduce interaction) to achieve:
- Speed: you just take notice of the N fastest responses that are consistent with each other
- Security: you are NOT trusting one node, but a bunch of them, just like bitcoind but without the blockchain download hassle.

[Bakemono]

Good question.
Until someone with actual knowledge of this matter answers you i ll make a guess 

For different instances of the same wallet to be synchronized.I mean for example if you have the same wallet (same seed)
installed in different computers.

[Mike Hearn]

What Electrum does requires a full index to be built of the block chain. bitcoind does not support this out of the box because it's unnecessary for Bitcoin's regular operation.

If you want a lightweight client that connects to a regular bitcoind, you need to use a regular SPV mode client (Electrum is not quite the same thing), like MultiBit. It doesn't use JSON-RPC but rather the P2P protocol directly.