CMcC Advocacy for Tcl is a fine thing, widespread Tcl popularity would undoubtedly bring considerable benefits, but I think it's worth considering the benefits which currently accrue to Tcl because its usage is not as widespread as (say) Perl or PHP's.
That's not to say I think we should stop trying to popularise it, merely that we should occasionally reflect on what sort of popularity we'd choose.
The long tail
One thing that widespread popularity inevitably brings is a long tail of out of date users who absolutely refuse to upgrade, but do want just that one bug fixed. This tail can slow the evolution of a language through effort expended backporting bugfixes.
Commmitttees
We love to hate the idiocy of some of the W3C recommendations, but to some extent they are an inevitable consequence of popularity. Once a thing is popular, and people/corporations have spent real effort/money using it, they acquire an interest in distorting future development to maximise their ROI.
Negative Normative Standards
Related to both the preceding points: imagine if you had to pull out an API book everytime you wanted to use Tcl or Tk. Tcl has a minimum of language lawyering, but as applications in Tcl became more popular, the interfaces between them would tend to become more like DMZs, no-man's-lands littered with historical decisions (aka 'mistakes'.)
Slowvolution
The dragging tail of backward compatibility, the byzantine operations of committees, the battlegrounds of negative normative standardisation all tend to slow evolution.
Summary
Popularity has merit, but it may be that we've dodged a bullet in not attaining world stardom.
Donald Arseneau This is like the terror felt by dog fanciers whenever a movie comes out featuring their favourite breed. What can happen is a huge demand for puppies by people incapable of handling an animal, filled by breeders who don't mind congenital defects.
As much as we can laugh at the people who claim, say, Java is the answer to everything, just think how much worse it would be if the same people were claiming and demanding that Tcl be the ultimate and complete Answer.