I think it is one of these things were you won't know if you need them...until you need them!
I would say buy them. Then at least next year if you need them plus more you have next years allowance and if you don;t get round to using them for a while then hey ho, at least you didn't have a shortfall.
I have never done an MFU before so hadn't considered that it would help cover an MFU once you take in to account the miles earned on a W/T booking. In that case £255 is a good price to get to J.
I added an EI H Class to my recent run to BCN/AGP for £129 which earned me the OW Bonus 15k miles. I thought that was good in comparison to the £255 BA charge. I am still undecided if I will buy the Miles. I have 5 on the household account so for £1275 I could buy 75k miles which I could use with my Fathers Amex voucher and get 2 J tickets for us to the US next year. That is still good value. I f I price the extra 25k Miles and the same cost then I am getting 2 J Tix for around £16-1700 which is half price.