I remember when they started paying transaction fees to miners. There was no "simple" way to do it under CPPSRB, so they decided that for the time being, the "worth" of a single stays the same, that is, the block reward divided by difficulty, but both the block reward and the transaction fees go torward paying the share log.

Thus, currently, Eligius paying out shares a little faster than it generates them, so that over the time, the unpaid share log would reduces in length. This is not sustainable in the long term, however, that log is so big that it will be many decades before the entire share log is payed like that. (Unless transaction fees increase dramatically) But when/if it happens, another way of paying the transaction fees will have to be devised.