Either your agent is reliable or not, if he is not, the number of days won't influence the outcome... it will be a bad outcome if you have an unreliable agent no matter if he keep the passport 1 day or 30 days. The issue here is that you are lacking experience in regard to agent so you are saying a bunch of stuff that are either irrelevant of flat out incorrect.
You don't seem to understand that you will need to send the agent your passport for a 2nd time went the under consideration is about to end, so it's more mailing and waste of time. Also, most agents are not in Bangkok. You have no idea what you are talking about, lol! There are 5 families in Thailand running the different agencies worth hundred of billions of baht. It's a huge industry run by people you will never heard of. Agents are middle-men, they are not the one who run the show (except in Pattaya and CM where agents work directly with the nearest immigration office).
I was simply answering your question on "Why agent keep passport for a month?", it's standard procedure for them to do so. Saying "BS... blabla, I've done this and that on my own!", is irrelevant because you did not use an agent. Why asking a question if you wont accept the factual answer?
There is a 3-4 weeks under consideration even if you use an agent. So passport stay with them or with immigration on a pile somewhere waiting for their turn to get the yearly extension. It's been like this forever.
This group is own by the biggest agency in Thailand (Thai Visa Centre), you can ask them... they will answer the same thing. You are confused between using vs not-using an agent. You can read my response to Jim Stuart above.
What you are describing is without the use of an agent, but if you use an agent, the passport stay with them because after a few weeks u get stamped for the full year. So yes, the under consideration still apply, but you don't need to travel twice. A lot of people use agents from other provinces, as some provinces don't have reliable agent. Just because you do not understand what agent do, or don't use them, doesn't mean it's dodgy. Maybe ask question instead of making judgment :). The owner of this group is an agent himself.
It's standard procedure for "under consideration". Most likely he is getting a year long extension so he need to wait 3-4 weeks to get his passport back.