Yeah. The illegal alien problem was a hell of a lot more manageable than the current mess is.
No system is going to be perfect.
What's occurred since 1965 isn't remotely close to the type of seasonal agricultural work that you're hearkening to. It's a free for all to hire illegal aliens.
Because of the 1965 law.
I'm skeptical that your aquaintances, who've participated by default up in the free for all, which is now the current custom, would be any more inclined to enforce the law against employers after a mass legalization via a guest worker proposal.
It would bring the overall posture of the law into closer accord with long-established (i.e., since 1848) custom.
Why should employers comply?
1. It's easier to be legal than illegal.
2. It's more likely that those who are illegal will be detected and caught (illegal activities will be more noticeable).
3. It's more likely that they'll be convicted.
I've seen nothing to suggest that hiring illegal aliens will be any more difficult after a legalization than it is now. It's really a stretch to say that it's difficult to know who's currently hiring illegals.
A real problem that advocates of the President's proposal have is that they ask for a rather blind trust that enforcement measures that aren't taken now will be taken later. If the President wants a politically pragmatic guest worker proposal, he'd be wise to incorporate the Tancredo plan and neutralize a ton of opposition on his right flank.