They need to pay taxes from the start, which is not happening if they are being paid under the table.
Plus who knows how many of them are actually trying to become legal? They could be happy going along for the ride as long as it lasts. Lots of ppl do that in the US.
Big deal if they not paying their taxes "from the start"... as long as they doing so and contributing like anybody else once legal that is all that matters. As for those who "happy going along for the ride" it easy to find them out, is not like Trinidad THAT multi-ethnic that we can't figure out which chinese in the country legally from who not.
The overall point isn't to say open up the doors to everybody, but to show how ridiculous it is to demonize a class/race of people based on the fact that they're illegal immigrants. Using your US example, there are many legal residents who are engaging in criminality, and there are many (I would even say most) illegal immigrants more concerned with keeping their heads down and working hard than with committing crime.
Some ppl will not care about the monetary benefits to the country of illegals paying income tax, and for those I could see why it's not a big deal if illegals don't pay taxes until they are legal.
However, infrastructure and services have a cost, and they should contribute.
And I am not confident about those who are set on remaining illegal being found out. Enforcement of laws is a big problem in our country.
In the US:
Illegals who decide to engage in further criminal activity is a whole other topic.
Of the illegal immigrants who just want to work, the only ones who pay taxes are those who steal a social security number. So on one hand they paying taxes, but on the other they are engaging in and increasing the demand for identity theft.