Since a few versions, fpc provides support for generics. It allows the developer to save some typing and also improves type safety at the time that prevents unsafe typecasts.
Unfortunately it's benefits is not for free. Every time a generic is specialized, the whole implementation code is copied into the unit/program.
To be more clear, i created two examples that implements a list of a custom class (TMyObj): one uses a TFPList, the other specializes a TFPGList. The difference in usage is that the former needs a typecast.
I compiled both with fpc 2.6.0 under windows. The result is a difference in executable size of 2Kb, the generic version being bigger. Than i looked the generated asm: the code to use the list classes are the same, the difference comes from the copy of implementation of TFPGList.
Many will say that code size is not a issue anymore given the availability of big hard drives, but i still think that is a good practice seeking smaller code. Regarding generics, it should be used, IMHO, when benefits are clear like classes that are instantiated many times in user (programmer) code and avoided in internal structures of e.g. RTL or third party libraries.