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As long as there are other people contributing, why should I do it? And if there is no other people contributing, then why to hold Monero?
That's exactly the argument why I thought it would have been a good idea to fund marketing and development from the block rewards, not from the pockets of bagholders.
I think Darkcoin has that sort of approach and I consider it good for long haul not to be dependent on the generosity of bagholders.
There are at least two very valid reasons why diverting a portion of the block rewards to development or some other project such as marketing is not a good idea at all.
1) Regulatory compliance.
This is the elephant in the room in crypto currency and Monero is one of very few crypto currencies that has avoided some very serious regulatory compliance risks. The critical aspect of this is the guidance that was issued by FinCEN in March of 2013.
https://www.fincen.gov/sites/default/files/shared/FIN-2013-G001.pdf. In order for a crypto currency to be compliant while at the same time avoiding the requirement of registration as a Money Services Business, and consequential AML/KNC requirements, the crypto currency must qualify as a "De Centralized Virtual Currency" under the guidance. This means that the block rewards have to go in their entirety to the miners and cannot be diverted for other purposes such as to fund development or marketing. If the block rewards are diverted in such a manner then those who are administering the diverted block rewards would be very likely be considered to be "administrators" or "exchangers" under the guidance, be required to register as MSBs and be subject to AML/KNC reporting requirements among other requirements. Ripple has already been subject to regulatory enforcement over this very issue. While I am not aware, at this time, of any regulatory action over this against Dash, the risk in that case likely remains very high. While this is specific to the United States, the reality is that the United States is at least for the time being, the world leader in financial regulation in particular because of the reserve currency status of the United States dollar.
In summary one needs to keep one's hands out of the till, block rewards, in order to keep the regulator away.
2) Coin Security.
Unlike Bitcoin like coins, in Monero the block rewards are needed in order to maintain the security of the POW. One cannot rely upon transaction fees for this at any point in the future. The reason for this is the adaptive blocksize limit. This by the way is also the reason why the tail emission is so critical to the security of Monero.
In summary one needs to keep one's hands out of the till, block rewards, in order to keep the 51% attacker away.
Edit: I should also point out that even in Bitcoin like coins, fees alone may not be sufficient to secure the coin in the absence of a block reward.