I suppose the argument against could be that you are using the privileges of your professional licence to instruct.
The fact that it is aerial work rather than public transport does not diminish the fact that it is a demanding and tiring pastime, whether you consider yourself fatigued by it or not - you are not 'resting'.
The hours count towards the grant of an ATPL (if you're a CPL holder) just as do those gained by flying as a line pilot for a commercial operator.
I know that the airlines I have flown for frowned, if not outrightly prohibited, employment outside the airline as a flying instructor.
That of course is the other issue - you are working (employed) by another flying organisation and, flying hours apart, that arrangement would not be acceptable to many airlines, as it not in other industries. It is not usual to moonlight for other companies when employed full-time for another, whether you are paid for it or not.
Whether this situation would be different for part-time employees is another issue.
Imagine a lawyer or accountant being employed by one company and then being put out that his employer frowned upon him working for another at the weekends! That would be nothing to do with hours worked or fatigue.
I appreciate that 'recreational' flying has always been seen as being separate to 'commercial' flying but, apart from anything else, must be seen by an employer as increasing his risk if pilots regularly fly on their days off rather than taking rest.
I also appreciate there are just as strenuous occupations outside flying which are just as tiring and fatiguing - however in these instances there is no direct relationship to your work as a 'pilot' nor your conditions of contract as an 'employee'.
KR
FOK