Slot cash helps if you have a top 10 pick or have a plan of attack to nab college commitments by buying them out over priced.
It helps regardless. You might not be able to go after the absolute best guys who are hard commits but there's plenty of guys who if you have extra money to spend are worth the gamble. Cubs top 30 comes out tomorrow so this is a bit dated but the 2017 list on mlb.com finished with Steele(5th round picks $1 mil bonus) at #10, D.J. Wilson(4th round pick $1.3 mil) at #12, Estrada(6th round pick $1 mil) at #19 and Bryan Hudson(3rd round $1.1 mil) at #27. All of those players are roughly $750k or so over their slot and you'd have inferior prospects in the cubs system without that additional money. And obviously that's to say nothing of Cease who thus far was their best gamble. And of course you have Sands who thus far just hasn't panned out.
Are you going to build a franchise around any of those guys? Of course not but they add significant system depth. The fact of the matter is after the start of the third round you're rarely getting more than a league average player at best unless you pay them significantly more. For example, Hatch is probably best case a #3/4 starter. He's ok but you're not going to sustainably build much with guys at his level. However, with additional slot money even as little as $750k you can turn 4th/5th/6th round picks into talent level of 2nd/3rd round picks. And when you're talking about a purely average draft having maybe 3-4 guys out of the top 10 rounds who feasibly could reach the majors adding additional guys to that is a big deal.
Imo the true talent is in the international field and that should be the plan of attack. Gaining resourse there makes more sense as that return has historically been better in general. The draft in general the rule is fail and the exception is success.
This isn't really accurate either. Historically IFA has been far more of a gamble. Don't believe me? Look at the top 30 IFA's on mlb.com from 2014. Anderson Espinoza is the only great name. 2013 looks better because the cubs pulled out Torres and Eloy and the Sox got devers but that's an atypical year. The best of 2012 top guys is probably Richard Urena. Now sure you'll have under rated guys like Castro who sign for nothing and pop up but those are far harder to find.
Also you can't "gain resources" in IFA anymore. Everyone starts with the same pools. While true you can acquire more via trade you have to have other teams part with that and even then there's a limit. That's not to say IFA isn't a way to bring in talent but it's far less reliable than you're making it out to be.