> > movff PORTx, PORTB
> 
> So what's wrong with this?
> just the fact I'm using PORTB intead of LATB?

It very much depends on the circumstances. The port pins are the
interface between the s/w and the external circuit, and it pays to
be aware of potential real-world problems

http://www.piclist.com/techref/readmodwrite.htm

These problems generally arise from not waiting for signals from or
to the external circuit to stabilise at the expected logic state

For example if you had a slow-ish clock, say 32kHz, and were
driving or reading TTL chips, then probably fine to use PORT. A
40MHz clock driving or reading lines with significant capacitance,
not a good idea to have port modifications in quick succession

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