I have noticed on some other sites it can lead to "if you like my photo then I will like yours" but in general I think it's ok and thus fine with me. 
This is actually a good point, in the past it was rather irritating when this was done with comments to each others photographs, especially when the comment was restricted to single syllable replies like "wow". It created a group of members that mutually made comments and thereby created much more views to the shots than these would actually have deserved on their own merits. Likes in the current systems seem to hardly influence the number of hits.
The alternative would be to opt out of the possibility to receive, or even to give likes but perhaps it does not have go this far.
