Looking at the numbers Lib Dems haven't a hope in Bury anyway (or, perhaps, most other places for that matter). But here's another opportunity for me to rant and rave about the flaws of FPTP -- shouldn't have given it to me, TTT!
(Quick version: Something something spoiler effect something something "clone-negative" something something voter-splitting something something plurality voting systems suck.)
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But in Bury it probably won't matter anyway; no local Lib Dem support to speak of, and even if you assume that there's no change to Labour/Tory support and that all Green voters and Lib Dems migrate to Labour... there's a whopping great 5k voters from UKIP who are mostly going blue. So it'll be in vain -- but still, imagine how annoyed you'd be if just another couple-hundred people had switched from Tory to UKIP, letting Labour in here.
What I think the Left are overlooking in their "progressive alliance drive" -- that is still worth aiming for all the same -- is that there's a similar vote-splitting on the right. Or, at least, there was -- but UKIP voters are going back to the Tories in their droves, and the sum of Tory + UKIP is currently outdoing what the Left can muster together.
Getting behind just one candidate is worth it, all the same, if there's no meaningful difference between the candidates and dividing the vote lets the opposition in. As I say, the missing piece is that this also works the other way. And apparently, voters on the right are somewhat better at just doing the necessary "unsplitting" of their own accord. In 2015, UKIP got about 13% of the popular vote (and a paltry 1 seat to show for it); I'll be amazed if they manage even half of that; and it's (almost) all going to the Tories.