1) Obviously the handling and ride won't be quite as good in comparison to replacing the shocks with a good aftermarket.
2) Eventually you'll need to replace the shocks due to normal wear and tear this will probably be accelerated by using stiff lowering spring. Replacing the shocks requires removing the springs. If you're going to do one you may as well do both if you have the money.
ive got eibachs with pretty worn out stock shocks on and its really not that bad. i dont road race or do any autoXing or anything so perfect handling isnt the biggest priority on my list. if you have the money i would go ahead and get some aftmarket shocks at the same time, but if you dont have the money its not too big of a deal to go back in and put new shocks in, it doesnt take that long to do.