it is all about battery. n900's unit is designed so that your device can use the gps longer than couple hours. some external gps units have approx half the battery capacity n900 has I'd quess so imagine giving 50% battery capacity away just to get better gps, would that be ok?

just use the gsm for couple seconds, then n900 outperforms every single sirf III equipped unit when cold started 10 to nil.....