Some of us take the heretical view that monitoring a competent handflying pilot in a situation of reasonably varying workload might be easier than monitoring his/her management of the automatics ?
Points to note -
(a) handflying I can compare what's going on to what I might be doing were I PF. Deviations are fairly obvious and, where warranted, can generate a between pilots query to resolve.
(b) with the automatics, I have to keep up with what buttons are being pressed and tie the sequence into a flight management strategy. Furthermore I have to keep up to speed with what the various annunciator indications are to satisfy myself that the button pressing has been accepted by the box... as well as doing all the other PNF stuff which might be useful.
(c) doesn't matter whether handflown or automatics .. if the workload gets silly, bug out and try again.
(d) it is a lemma that a competent pilot has to be competent at both modes of operation ..