The problem is that the two systems (queuing then mindlessly wandering around with heavy trays vs finding your table first) are incompatible, and that everyone should only use one.
Since the fairest and most efficient is to find your table first, ensuring everyone who orders food or drink has somewhere to sit, everyone should thus adopt this system.
It costs more to eat in rather than take food out, so your 'contract' with the cafe is that that cost allows you to sit and use their facilities. It makes no sense whatsoever to pay this price if you are not sure you will be able to sit down straight away.
It really is more efficient this way - you enter the premises, check to see if your group can all be seated, if yes you settle your family and one or two of you go to order, if no you leave and go elsewhere. Everyone knows at the start of the transaction that they have a suitable table.
The alternative is to have the queue artificially inflated by everyone queuing needlessly (in our case 5 of us when usually two of us queue), making it hard to form a realistic picture of how many tables-worth of people that is. Are those 20 people in queue 5 lots of 4, or 10 lots of 2, or perhaps a few singles and lots of 3s? By contrast, when people are choosing a table first, it is self-evident what spaces are left.
Then, having made the queue look horrendously long, the five of us are expected to trail around the cafe looking for someone to sit. Oh yes, and we are expected to ask others if we can share their tables, regardless of the fact that the sole reason we have entered that establishment is to sit together and not sit with strangers.
You see, if everyone adopts the queue-first approach it still doesn't work because you still take the risk of getting to the end of the queue, paying, and still finding there isn't a table to suit your party. By contrast, if everyone adopted the table-first approach it works seamlessly because you know from the outset whether or not there is space.
I think it's a minor problem anyway, to be honest. Very few places have a queue-first rule, and the cafe owners or managers I know all prefer their customers to find a table first, so for them it works best too.