yes possibly - though in my experience explaining that to a police office (with no disrespect meant - most are tech illiterate) could be difficult.
Then you have the huge issue of explaining that to the jury; the defence would probably still have a field day.
And finally, certainly here in the UK so this might not apply elsewhere, there is quite a strict mandate for handling evidence. Returning the computer and keeping the HDD's would almost certainly break those guidelines.
It's really illogical all round; the main problem being computer evidence is still treated with suspicion by Juries (probably due to technical misunderstanding) and greeted with delight by defense teams (who will take joy in asking obscure, mundane and irrelevant questions :D just to derail things)
Finally: complex investigations might really take months to complete (as in 8 or 10). And then going to trial can take upwards of 2 years.
EDIT: with all of that said I strongly disagree with the police actions in this case.
Not giving a list of seizures, even within a few hours, is definite grounds for massive legal action here. It would be unheard of, frankly.
Not giving computers back after, say, 5 months where the case was unproven or the computers irrelevant would also be wrong. In the case where information was found but no trial forthcoming stuff should be returned within 2 years.
I wont comment on the legality of the raid though; we are only getting one side of the story and it could be there is evidence to have raided them. We also haven't seen or heard about the warrant used; so we cant really speculate on the legality of the seizures.