The 5 -100's we have at BA (G-BUSB-BUSF) are pretty useless as a commercial proposition if the truth be told. To show you what I mean, the weights we use are (-200 CFM in brackets for comparison).
Max ZFW 59,000kg (60,500)
Max Land 63,000kg (64,500)
Max T/O 68,000kg (73,500)
I have had had a situation where wx at LHR had me planned to land at LHR at max ZFW and at max landing weight with 4,000kg fuel (about 1100kg reserve + (say) 1600kg MAN div giving 1300kg contingency + extra (1300kg is about 35-40 mins holding capacity). That gave a max trip fuel of 5000kg, or a range of only 2 hours. In other words Oslo, Prague, Vienna, Madrid are max range in these circumstances. If you are going somewhere with a more distant alternate, then they really are little better than useless.
In comparison. the -200CFM's (and IAE's) allow you to all of the above and carry an extra 5500kgs fuel (extra 2:15 range).
HTH