Three line whips aren't particularly common, since not all debates end with a division (votes), and the party will not have a policy on all matters. Moreover a three-line whip instructs the MP that they absolutely have to attend, excluding the possibility of dealing with other political affairs, whereas for lesser whipped options an MP might be able to "pair" with a member of an opposing party (i.e. both agree not to vote, thus freeing the MPs to conduct other business without changing the result).
Obviously three line whips are far more common for more important votes though, which obviously the "Brexit bill" counts as. Some points made the Labour party whip particularly notable in this case though were that:
- an opposition party was whipping for the government position on a contentious issue.
- Labour voters are estimated to have voted to remain in the EU by about 65% to 35%, however roughly 2/3 of constituencies which elected the current Labour MPs voted to leave.
- Some of the 1/3 of Labour constituencies which voted to remain did it by large margins, including a ~ 75% to 25% vote in Islington North, the constituency of the Labour leader Jeremy Corbyn.
The popular expectation in the media therefore had been that Labour would either offer a free vote (allowing their MPs to vote their consciences) or to whip weakly, thus avoiding the awkward disciplinary questions of what to do when individual MPs rebelled.