Yes, but many systems have some sources of true entropy, and if they don't, you can always introduce them yourself. On-chip temperature sensor. Cooling fan speed. RTC combined with some outside (not clock-triggered) signal source. Pushbuttons. Network. Ringing from fast edges. The problem is the entropy may be poor (i.e. the source gives you "1" in 90% cases, "0" in 10%, so for true 50% 0/1 output you need 7 or so bits read) and the bandwidth may be low (1 such bit per second) - so if you i.e. need to encrypt a data stream at 56kbps, they are useful, except for seeding/salting.