Why did Minerva McGonagall suggest that Hogwarts only be guarded/protected basically, and not to their best efforts?
Here's a line from Deathly Hallows, just shortly after Severus was chased away by the combined effort of McGonagall, Flitwick, and Sprout:
‘Professor, we’ve got to barricade the school, he’s coming now!’
‘Very well. He Who Must Not Be Named is coming,’ she told the other teachers. Sprout and Flitwick gasped; Slughorn let out a low groan. ‘Potter has work to do in the castle on Dumbledore’s orders. We need to put in place every protection of which we are capable, while Potter does what he needs to do.’
‘You realise, of course, that nothing we do will be able to keep out You-Know-Who indefinitely?’ squeaked Flitwick.
‘But we can hold him up,’ said Professor Sprout.
‘Thank you, Pomona,’ said Professor McGonagall, and between the two witches there passed a look of grim understanding. ‘I suggest we establish basic protection around the place, then gather our students and meet in the Great Hall. Most must be evacuated, though if any of those who are over-age wish to stay and fight, I think they ought to be given the chance.’
‘Agreed,’ said Professor Sprout, already hurrying towards the door. ‘I shall meet you in the Great Hall in twenty minutes with my house.’
Why did she only suggest basic protection around the place? Did she forget that Harry needed "as much time as possible," which she said clearly just a few lines earlier? Wouldn't it be more useful to have a barrier that will last longer so Harry will have more time?