The key elements of machined vs. cast reels have been handled here pretty well by people with more technical expertise than I have. I would just like to add that there are plenty of "catch phrases" that get used in the marketing end of the fishing industry like soundbites and talking points in Washington DC. Machined vs. cast sort of fits in that same category nowadays. Not too many years ago, it was a general guideline that cast reels were usually of inferior overall quality. That was reflected in radically lower price points, too. But the manufacturing technology has changed in the past decade or so, and several reel mfgs have taken advantage of these improvements to bring some very good cast reels to the marketplace at a price point that is more-or-less half way between the old "cheap" cast reels and the quality machined reels. Today, whether a reel is machined or cast really is no more indicative of the overall quality of the reel than the phrase "high modulus" means that a fly rod is made of superior materials. There are cast reels that are being deep anodized with great success. The new polyurethane coatings are proving very effective. Heck, even some of the high-impact carbon polymer reels are pretty darned good if "pretty" isn't important to you.
I own a couple of cast big game reels that I use in both salt and fresh water. They're great. The mfg said, "Now, be sure to rinse these off right away if you use them in salt water. They're not fully machined from bar stock, you know. They're fine for salt water use, but they probably won't take much abuse. If you leave them sitting around with salt water on them they will probably rust." Well - I don't actually expect my gear to take abuse. I take good care of my property. Everything is always supposed to get a good rinse after exposure to salt water. I have owned 2 pure 6061 aircraft aluminum bar stock machined fly reels that rusted after being fished in saltwater even though they were rinsed promptly. They didn't have good stainless steel screws in them! The Devil is in the details. I have used these cast reels for a couple of years now (including spare spools) with no signs of wear and tear. The machined reel in the large size comparable to one of these cast reels bears an MSRP of about $400. The cast reel has an MSRP of about $130. And it has a better drag in it too.
The longer you do this stuff...if you're paying attention and eavesdropping frequently...the more you come to realize just how much of the "conventional wisdom" is actually just marketing buzz or commercially driven. It is about catching fishermen more than it is about catching fish. That's what pays the bills. There are some fantastic machined reels out there. I am in no way saying that a great machined reel isn't worth the extra money! SOME of them may be. I have a mix of both machined and cast reels, and will probably acquire more of each. I also have some polymer reels. (talk about a reel that is salt water friendly!) But there are some very high quality products out there nowadays for the budget-minded fly angler. You can have high quality gear that won't fail you without breaking the bank or getting served with divorce papers.