Why? Because no one has any way of knowing what else it can do.
Unless they've swallowed a metalmind. If they know that there is a power there to be accessed, but can't manage it, burning lerasium simultaneously might rewrite their sDNA to gain access to that power. In the case of a Twinborn or hemalurgist using his own metalmind, this might give them full Allomancy, full Feruchemy, or both, allowing them to fully access the variations of their power. A full Feruchemist would just get full Allomancy, like Rashek did.
But what about an Allomancer who was not a Twinborn or otherwise possessed of Feruchemy? What about someone using another person's metalmind? What if Vin had burned lerasium while trying to access Sazed's metalmind? I think that lerasium might be able to rewrite sDNA to give a sort of skeleton key Feruchemy, a way to unlock metalminds from other Feruchemists. A Skeleton Key Feruchemist might be able to steal others' metalminds, to tap memories he never experienced or even become other people. It would be interesting, to say the least.
1. That it only opens up access to that one person's metalminds.
2. That it opens up the other person's at the cost of your old ones.
3. That it allows the use of any metalmind created by any Feruchemist.
All three could have their potential story uses. A reasonably steady supply of lerasium with #1 could turn a series of seemingly mundane thefts into a high profile crime of the century as the thief (or gendarme) closes in on the final goal with the stolen power. #2 could be used by a group to create communal metalminds, leading to a potential group mind and hive society. #3 is just out there, but I still think it's cool, and it's the one that requires the least amount of lerasium to kick off.
Edited by Eric, 29 April 2012 - 10:11 AM.