Here's an absolute worst-case scenario if those barriers are lifted:
A person - spammer, troll or someone else acting in bad faith - creates a few dozen accounts. Each account makes a couple posts, and the other accounts upvote those posts. Each user has 40 votes they can use per day; that means that each account could, in theory, deliver 350 reputation points to the other accounts. That gets really bad really quickly.
However, if you keep some rep requirements - 15 to upvote, 50 to comment - then it makes it really hard for this person to do anything. They'd need an account with at least 15 reputation from the start, and even then, they'd have to dish out two upvotes to each account to just get them voting privileges. The 40 vote cap limits how quickly this can propagate.
Now, mods can dismantle a network like this. We've got the tools to see patterns and important information. But it takes time to fix this mess - hours, maybe days to make sure we've taken care of everything. We'd have to check all the accounts to look at IP addresses, destroy them individually, delete all the posts, etc. Calling in a Community Manager could make this quicker, but they've got enough on their plates already without having to shift into crisis mode. On some of my sites, we have problems on a daily basis without having to worry about webs of spammers ready to strike at any moment.
And this is all if we catch this as soon as it starts. If the troll/spammer/puppeteer manages to play their cards right, the network can form quietly over some time without anyone being the wiser. The system, by setting these limits, starts fighting the puppeteers simply by making it hard for them to do anything. And yes, it makes it a bit harder for users to get started. I sympathize. We all started from the same spot. If you're acting in good faith, you'll be able to gain privileges with a bit of effort.
But if we take away these barriers, things go south real quickly.