As long as a person believes that there is only one God & Muhammad (may peace be upon him) is his messenger then he or she is a Muslim. When inviting people to Islam, that is what scholars focus on the most & upon other topics too alongside those they are questioned about. However, that does not encompass every teaching of Islam, anyone with an ounce of logic should understand that. Even people who were born Muslims do not know every ruling & legislation of Islam. The same applies to followers of many other religions. Their motivation does not matter to ordinary people because their conversion as per our & every other religions' beliefs is to follow the right path. As long as they follow the religion after conversion, why should we doubt that their conversion wasn't sincere? Even if we do doubt their sincerity, then that can't be used to judge them due to the fact that we do not & can not know their true intentions. People can convert to any religion they want for whatever reason they want, most people simply do not care. All the textbooks in the world pertaining to Islam aren't always available free of cost, & neither is their education. Needy people are applicable to receive charity regardless of them being new converts or born Muslims. Zakat has to be paid to the poor, it isn't used to fund other activities. It's up to the receiver to decide how he or she may use it.
Which incentives & benefits are most converts receiving? A hell of a lot of people who convert get no Zakat whatsoever because it is reserved for the needy alone. Does that continuously needed to be repeated? Neither is any payment made in the form of bribery either. Middle class converts to Islam receive no charity upon conversion. Furthermore, people may acquire charity even if they are non-Muslims although that obviously won't be the compulsory Zakat. Note that Islamic governments have to care for all of their citizens regardless of religious status. Besides, even needy converts can't rely solely on Zakat to survive & paying them or any other poor Muslim is solely dependent on the giver. Thus, it is illogical to assume that every convert changes religion for monetary benefits, & it's highly unlikely that charity alone can be enough of a benefit to convert. Of course, it is always better to have only those people convert that truly believe in the religion & judging by the efforts made by scholars that invite people to Islam, that's most likely the case. Opportunistic people can't always be controlled, & as stated earlier, the majority of the populace does not care about the number of Muslims. Islam is already one of the largest religions in the world. Those who convert do it for their own sake, not to please existing Muslims. I haven't bragged about Islam being the fastest growing religion on this thread. God does not have an issue with providing charity to the needy as per Islamic texts, thus charity should be acceptable to Him. This isn't bribery, anyone that exploits the system does no harm to the religion whatsoever. Another thing, as per Islamic beliefs, no human is responsible for another's actions so already existing Muslims have nothing to lose or gain by another's conversion.