I disagree, I think its important to have acknowledgement, but it merely needs to be sent via email, it is much more reliable than Royal Mail and majority check their email at least every 3 days. I hope uni's would send interview invites via email as well as posts! Moreover, acknowledgements contains important information which candidates should be aware of eg when interviews take place(eg a general timescale), assurance that they have received applications, a decision will be made in due course,as well as contact details and that the uni have received ALL information.
If they didn't bother with acknowledgements, their emails and phone lines would be jammed with candidates who are worried because they haven't heard from the uni in many months and are at unease, especially if they come to forums like this in which people post they received a letter inviting them to interviews.