As far as I know there is no need for actuarial advice on a trust apportionment, if all beneficiaries are agreed on a split of the fund.
However, having one does protect their advisors if it subsequently becomes clear that one of the beneficiaries got less than their interest was worth.
Problems may arise if the life tenant dies soon after the apportionment. It may happen purely by chance. There are medical conditions that are completely symptomless which might cause an apparently healthy person to suddenly just drop dead. An aneurysm is almost always symptomless until it bursts and someone who might be apparently perfectly happy and healthy one minute is dead five minutes later. There might well be warning signs with strokes but they still frequently come as a nasty surprise.
The problem might have been unknown or even unknowable but the remaindermen are likely to feel aggrieved if they would have got substantially more, if the trust had continued. This might lead them to investigate whether the apportionment was in fact fair. Hindsight is wonderful, but human nature is what it is.
As regards cost, administering a trust costs money one way or another, and I would not expect the actuarial fees to be that large in comparison the annual costs being saved or indeed in comparison with the cost of the legal work involved in winding up the trust.
Ian McKeever & Co Consulting Actuaries