One potential downside people often point out about using TextUML as a notation for UML is that it is yet another language to learn. But that is not really a well founded argument. TextUML is not a full-blown language, it is just an alternative notation for UML, the de facto standard language for modeling. The language semantics of UML are defined in terms of an abstract syntax, and even though the specification includes sections about the graphical notation, it clearly supports the idea of alternate concrete syntaxes.
Moreover, we really shouldn’t care much about the actual notation being used, be it the standard graphical notation, TextUML or any other textual notation. I surely don’t. Regardless what notation you use for creating UML models, in the end there is only one language. All you know about the semantics of UML model elements is still true no matter what syntax you choose (for instance, if you know your UML, you should easily become productive with the TextUML Toolkit by just glancing over the notation guide now and then). In fact, I predict a time where people will want to move between different notations across tasks and time. Also, in the same team, different people will be collaboratively creating UML models using different notations.
This will only be possible because supporting a new concrete syntax is much simpler than supporting a whole new language (by the way, that is the same reason why I believe UML-based DSLs to be a better option than homegrown proprietary DSLs, but I digress). Any reasonably good programmer armed with a parser generator and knowledge of the metamodel should be able to write a compiler for a textual notation for UML, and Eclipse makes it really easy to provide basic IDE features such as those you see in the TextUML Toolkit. Also, there are tools in the horizon such as IMP and TMF that will make these tasks really a breeze.
Meanwhile, you can stick with what is here today.