Some "drive-by" responses ...
1- If you are using Exchange for email, then AD is required. You likely are not using Exchange or you would know that, but I include it for those who may be considering this.
2- AD manages a "centralized authentication" system. You control users, groups, and passwords in a single place. If you don't have AD, you will likely have to setup your users separately on each terminal server, or have a generic user on each for access and use security in the application.
3- If you have other Windows servers, AD allows for straight-forward securing of resources on those servers in a single place (AD).
4- AD includes some other services (DNS, DHCP) which otherwise have to be managed separately. I suspect you may not be using them if the only Windows servers you have are the terminal servers.
5- Although not required, there is benefit to having the workstations in the domain. This allows for some (not comprehensive) single sign-on capabilities as well as significant control and management of the workstations through "group policies".
--> For instance, through GP you can control the screen saver settings, requiring that the screen saver lock the workstation after x minutes and requiring the password to unlock.
6- You might be a good candidate for Microsoft Small Business Server if you need email, file sharing, remote access and web serving.
I second the note about having two domain controllers. If you only have one DC and it fails, you are in for real pain getting access to things. It is (I believe) possible to have the terminal servers also be domain controllers, although I suspect many will not recommend it. In a small network like yours the DC workload will be insignificant, so it might work.
EDIT: in a comment s.mihai asked: "it's their interest to make us buy all we can. but can i be OK without AD ? local accounts, no exchange.... ?!"
Were I in your shoes, I would use the TS project as an excuse to add AD for the benefits, particularly on the workstations. But it sounds like your mind is made up and you want cover, so here it is.
ABSOLUTELY you can be OK without AD.