Dancer uses Felons as a way to mobilize Convicts' shield sidearm into offensive damage.

While the main trick of his strategy is to simply have enough Convicts to instantly turn any combat encounter into more Convicts, this doesn't really work if the enemy doesn't willingly feed the Convict ball; and they won't do that unless the Convicts emit some kind of credible threat.

Thus the Convict army need to be able to directly threaten the enemy, and the Felons are the way they do it.

I sense a philosophy here. Convicts, Felons, Dominatrices, Funnelwebs. The war must feed and finance itself, and that mandates self-funding units that can turn even otherwise lost engagements into a net advantage.

This is not "turtling" but "steamrolling": the convict ball moves onwards, tiling the world in mexes and eco, reclaiming all that stands in their way.

Of course, this works much better either on narrow maps where the Convict ball cannot be outmaneuvered, or in teamgames where there almost always is someone who's willing to feed the reclaim machine.

In my experience naked expansion was very effective vs. Dancer.
While the ball is scary, it requires 100% attention, so raids are either non-existent or ineffective.
This means you can stabilise map control in your favour and from then on, you just need to exert a constant trickle of pressure to keep Dancer in place while feeding less than your advantage.
A neat trick is to scout when he's starting the Dante/Funnel/cata and taking the opportunity to build your own (which may well be finished faster).