The pagers were used by Hezbollah, not Hamas. They are two different entities, and while it doesn't make any difference in the narrow context I'm replying to, it's really a basic detail that anyone voicing an opinion on the matter should know.
How is this argument different than defending the use of landmines?
From the Wikipedia entry about landmines: "The use of land mines is controversial because they are indiscriminate weapons, harming soldier and civilian alike. They remain dangerous after the conflict in which they were deployed has ended, killing and injuring civilians and rendering land impassable and unusable for decades. To make matters worse, many factions have not kept accurate records (or any at all) of the exact locations of their minefields, making removal efforts painstakingly slow."
Planting bombs inside pagers specifically used by Hezbollah isn't indiscriminate (unless by "indiscriminate" you mean "when they go off, they harm anyone in the proximity", but going by that definition everything with an exploding charge is "indiscriminate", yet only mines are banned). And obviously exploded bombs don't remain dangerous and aren't difficult to remove.