Seems a pointless declaration if that was the reason. They get no extra points themselves and just stop Middlesex earning one point. If it were the last match of the season, and they were neck + neck with Middx with one point the potential difference in the title/relegation race, there might be some sense, but surely not at this stage.
An extra 20 or so runs and half an hour extra at the crease might be the difference between a draw and a defeat if there are further rain interruptions.
You have to wonder about the state of the wicket when a side is out so cheaply, but Middx have started OK
On things like declarations, I remember Somerset once - around 25 years ago -declaring their innings in a one-day game at 0-0 (ie an immediate loss) as qualification for the next stage of that cup (B+H?) was to be on differential run rate if sides were tied on points and the declaration meant that their nearest rivals couldn't overtake them. I think they were fined, their captain heavily censured, and the rules changed