A good question, fully agree about tying up money, I use an agent and have done for many years. Ok if you happy to pay an extra 12.000 baht or so more.
Massive risk without it. even if you put money aside, could clear out your savings if a serious issue, There are companies that will pay out as long as you are honest in your medical history. I had an issue 4 years ago and my company paid out as per contract. If you have millions of baht you can put aside then ok, and 3 months isn't long but it only takes one situation. Maybe worth the risk, but a big risk in my humble opinion.