I do not think it is an issue if the person leaves for a higher paying job when the two positions are practically identical. That is what I believe at least since that is the position I am in as far as staying or changing jobs. I am 3 months away from saying I have worked for 2 years for my current employer. There is another employer that hires for the same work I do here, and they could offer me $30-$40k more in base pay. I have been studying the perks of staying and leaving for several months, and ultimately the increase in salary would outweigh the perks of staying. Considering I will continue to develop in the same field of work regardless of which employer I am working for.
That goes without saying if my current employer can up the ante then I will have no reason to leave.
If it is apples to apples and you are around the 2 year mark, then you can easily justify a 30-40k bump as the reason for your leave.
Typically, 30-40k bumps are not the same job so there are development criteria to consider.
The difficult question is whether you give your current employer the opportunity to match the competitor when an offer of 30k+ comes through the door. Some employers might tell you good luck, others might tell you to not let the door hit you as you leave, and others would be thankful for letting them have a chance to keep a resource that they are evidently significantly underpaying.
I think the decision of how to handle it comes down to relationships. You also have to be mentally prepared to walk out the door the day you bring up another offer.