"Normal" users should not be using bitcon-qt.
Well, it's the official client for one.
There's nothing "official" in Bitcoin, and the fact that you think there is shows an understanding issue.
Saying that bitcoin-qt is the "official Bitcoin client" is like saying Microsoft Outlook is the "official POP3 client" or that Firefox is the "official HTTP client".
Granted, it is the "reference implementation", the only one so far which fully implements the protocol, and as consequence, the one which actually defines it. That's different from being "official" though.
For two, the more people that stop using a "full" client, the fewer full client nodes we have, and the less secure the network is.
Network security is much more related to the amount of computing power behind mining than the amount of full nodes in the network.
It's unlikely that someone manages to DDoS or hack all full nodes at the same time, even if only solo-miners and pool operators were running full nodes.
And, please, understand: if bitcoin succeeds, it is just a matter of time until this happens (few full nodes). If that really makes Bitcoin less secure as you say, then you may say Bitcoin is not secure by design.