It is not good to consistently give a short talk after every Prayer because first, people may feel bored, and second, some people may think that it is an integral part of the prayer itself.
Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
It is not a good idea to give a short talk after every Prayer without giving people a break; for in this way people may be conditioned to think that it is an integral part of the prayer.
Indeed, this has happened in some communities in similar cases. For example, the collective supplication which some imams are in the habit of making after each and every salah, even though it was not done by the Prophet (peace be upon him) or by the early Muslims, nor was it maintained by any of the four schools of Jurisprudence.
It was introduced with a good intention in countries where people were new to Islam; thus, they thought it would be hard on them to learn and memorize supplication; so in order to facilitate their learning of transmitted supplications, the practice of collective supplications after every prayer was introduced.
Unfortunately, as people have become used to it, they are often under the impression that it is integral part of salah; so many of them now seem to think that prayer is incomplete without collective supplication. Thus, they ended up distorting the prayer that was transmitted by the Prophet (peace be upon him).
Therefore, it is advisable for imams to avoid establishing the practice of giving a short talk after every prayer.
However, there is nothing wrong if this is done occasionally. Let us never forget the Prophet’s saying (peace be upon him): “Pray as you have seen me praying.” (Al-Bukhari)