mobile core i5, by itself, is a capable cpu.
which model are you refering and what are you comparing the i5 to?
for example, compared to mobile core i3, i5 is advantageous. it has turbo boost, aes-ni support. but compared to quad core mobile core i7, the i5 is at a disadvantage because it has only two cores.
desktop core i5's disadvantage compared to desktop core i7 is that it lacks hyper threading and is usually clocked lower at stock.