That is a matter of choice. A B&B or small hotel is a business and different businesses make different business decisions.
The strongest argument in favour of using a self-hosted Bookings system is flexibility: you can change every aspect of your Booking system (or pay somebody to). The strongest arguments against using a self-hosted Booking system are cost and number of integrations.
Monthly price for 10 rooms
+/- EUR 110,- (excl. VAT)
+/- EUR 110,- (excl VAT)
The Room Booking plugin drastically reduces the costs of a self-hosted Booking system and makes a hosted Booking system seem really expensive. Whether it is depends on your situation.
How many of the 400+ integrations SaaS sollutions boast about do you plan to use?
What is your revenue? Spending EUR 1500 per year on a SaaS solution that is crucial for your business is not that much (probably less than 5% of your revenue).
You will probably pay at least EUR 6000 if you have something custom developed.
For any business it is important not to become dependent upon somebody else's business. For example, you do not want to rely on a single Online Travel Agency (OTA) to get bookings since that would give that OTA far to much power over your business (and demand even higher commission fees or even lower prices).
You are not happy with this plugin? Use another. Unhappy with webhosting? Find a new webhost. Unhappy with Booking.com? Focus on Expedia. Unhappy with your bank? Switch. The point is that you stay in control of your business and keep your options open. Do not become dependent on somebody else's business.
The one-time fee for the Room Booking plugin helps you keep recurring costs down to a minimum. So do the direct bookings on your website that this plugin makes possible. But the Channel Manager of the Room Booking plugin has limited possibilities and you will have to maintain the software yourself (or pay somebody to do this for you).
In the end, it remains to be seen which option is cheaper or when an option becomes preferable.