These days I was messing up with Electrum's API, and I discovered the obvious. We all are happy with it because it does not need to download the whole blockchain (~ 90 GiB) and is quite easy to use, too.
The point is this: the whole blockchain is needed anyway. Electrum's solution: to demand to "electrum servers", which are not bare bitcoin nodes, the responsability to hold the data for him. So: Electrum helds no blockchain on your PC, but it connects to Electrum server which has it, that can or can not be an active bitcoin node; Electrum asks just what it needs to know and the trick is done.
This may be a problem since it inserts another layer between you and the bitcoin server world. All in all, it not much different to use an online wallet: between you and the bitcoin blockchain there is someone in the middle.
When you use the bitcoin original sw, you become a node yourself, you connect p2p to many other bitcoin nodes, you broadcast a transaction or receive the last blocks to and from them. You can choose to mine, you can choose to open a port to become part of the bitcoin network, or you can tell it to just use tor proxy to fetch and send data, and stop.
Electrum is open source, while a web wallet is not. An Electrum server is something we don't know nothing about, too.
What do you think?
p.s. point is: we do need bitcoin servers, we do not really need Electrum servers (they can log everything they want, btw)
It's never failed me, I will continue to use it.