Well, publishing server side code IS actually important if you provide TLS encryption - in other words, if you encrypt data on the way to your servers, and then from your servers to the recipient. Of course, if Telegram were exclusively based on end-to-end-encryption, your statement would absolutely be valid. But usually, you have TLS encryption in chats (except secret chats) and in groups by default. This means that, if we can't obtain the server-side code, we'll have to blindly trust Telegram to keep our data safe and refrain from spying on us. This is an obvious risk and thus I must say that, in my opinion, Telegram is still vastly inferior to apps like Threema, while of course being the superior choice compared to WhatsApp and Facebook Messenger.