If you re-read the part where Elrond agrees to let Merry and Pippen into the Fellowship rather than the two elf-lords he was considering, it's really rather amusing. What he actually does is relent, to Pippen's badgering -- pick me! pick me!-- and to Gandalf arguing for the importance of friendship over sheer might... as if friendship isn't something that is quickly forged when on travels, as it was between Gandalf himself and Shadowfax just before he got to Rivendale, and it will soon be between the elf, Legolas, and dwarf, Gimli, on their way to Mordor.
Elrond the great leader more or less goes, "fine, it's only the end of the world if you fail... take your two munchkins over my elf-lords, even as even if two elf-lords can't "storm the Dark Tower, nor open the road to the Fire by the power that is in [them]," they could at least come closer to that pippenmerry possibly could.
It's irritating that pippenmerry weren't given better reason, for as it is it really seems folly that Elrond didn't wave them and Gandalf off, reminding them all that just previously they only got away from the Ring Wraiths at Weathertop because they were beset by only five rather than the full nine of them... increments in power absolutely still count, even as the course is of evasion not brash confrontation of Mordor's total force.