First, the history of coalition governments under a first-past-the-post (FPTP) electoral system shows that, at the subsequent election, the perceived electoral choice is for the government (vote for the larger party) or against it (vote for the opposition). The smaller coalition partner suffers heavily from a loss of its distinct identity. So a coalition could only be based, so far as the Liberal Democrats are concerned, on a cast-iron commitment to a reformed electoral system being in place before the next general election.
Second, Mr MacIntyre answers his own question with his next words: "Blair is now impatient to gather together the collective anti-Tory forces while he is ahead". The worst way forward for British politics would be the combination of a single party containing "all the sensible people" on the one hand and the continuation of a FPTP electoral system on the other as, by definition, sooner or later some non-sensible party would win.
The better way is to embrace the pluralism inherent in a reformed electoral system, and recognise that there is at present, within British politics, a spectrum of five parties (six in Scotland and Wales), with the Conservatives fundamentally split between the English Nationalists on the one hand and a Christian Democrat-type strand on the other, and the Labour Party divided between "New Labour" and the "Tyrannosaurus Tendency". "Gathering together the anti-Tory forces" under a FPTP electoral system is precisely the opposite of that pluralism.