If you have all the LPAR set to 128 it will mean each will get an equal slice of the unsed CPU cycles. An example is if you had 4 LPAR's all set with a weight of 128 and each LPAR needed additional CPU cycles they would each get 25%. If you had one set at a higher weight if would get a greater percentage (but not all) than the others.
Basically you would get better benefit to set you more important
systems to a higher weight then you least important systems.
Bear in mind that even if set to a low weight (except 0) they
will still get a percentage of spare CPU cycles just less the
other systems on a higher weight.
An example of the weights I user are:
208 - Syste (Mainly VIO etc)
192 - Critcal production
160 - Production
80 - Critical dev
48 - Dev
9 - Loan/Temp LPAR's
Also remember weight only applies if it wants to use CPU cycles
greater than it already is entitled to/allocated.
hope this helps.