alright ... since i talked a little smack about cottered cranks over the weekend i feel like i should throw in my two cents. but first, i should clarify something. i give bike advise from the perspective of someone who knows enough to find the problems, but does not know enough to solve all of them. i also assume that people want to get on the road as quickly and as cheaply as possible. if you know lots about bikes and have time and money to burn, nothing i think of as a problem will be one for you.

now that that is out of the way, if the cranks are in good working order and you don't feel the need to change the chain ring or get into the BB shell to do some greasing, you may get many miles out of these cranks.

problems arise if you want to dismantle the cranks. assuming you can get the pin out in the first place, it will be hard to get it to fit back in as snuggly as it was before and, as someone mentioned, new pins of good quality are hard to come by.

i doubt that cottered cranks which are in good working order are mechanically inferior to cotterless cranks (esp. if we're talking about a beater) but this is only true as long as the pin holds up ... and since the pin works by being forced into a hole that is too small for it, the cranks will only work properly for the life of the pin ... which is relatively short since, in order to function, it has to be sort of smushed into place - and thus compromised.

if you have the inclination and money to change the cranks (and, most likely, the BB and spindle) try to make sure you will have the right threads to do so. i don't know how to figure this out for sure so i wouldn't do it. but that's just me ... cheap, mechanically disinclined and lazy.