I have lots of slip-on tub spouts in my building, and they're never a problem... IF you've got a decent slip-on spout.
In my humble opinion, THE BEST slip-on spout out there is the Moen 3931 for only about $13 each:
Type 3931 into the "Model Number" field and click on "Search Products".
Which can be ordered from any plumbing wholesaler or retailer who sells Moen.
There are three reasons why this spout is better than other slip-on spouts:
1. It has a MUCH BETTER system for clamping onto the copper slip-on pipe
2. It doesn't waste any water, and
3. replacement parts are available for it.
Lemme elaborate on each of these points:
1. Just like other slip on spouts, the Moen 3931 requires that you tighten a 5/32 inch hex cap screw to install the spout.
BUT, tightening that cap screw does not result in a set screw impinging on the copper pipe. Instead, turning the cap screw causes two curved yokes to be pulled together, clamping the copper pipe between them. The upper yoke straddles around the copper pipe and has a right hand thread in it's bottom that causes it to be pulled DOWN as you turn the cap screw. The bottom yoke has a left hand thread in it which causes it to be pushed UP instead. The curvature of each yoke exactly fits the OD of the copper pipe, so the clamping force is spread out over a much larger part of the circumference of the copper pipe, so it's very much less likely to damage or crimp the copper pipe than the traditional set screw is (which applies all of it's clamping force in one small area).
2. If you buy a Moen diverter spout from Home Depot, you've bought a water waster. That's because when you pull the diverter up to have a shower, at least 33% of the water will leak out the tub spout instead of going through the shower head. The Moen 3931 doesn't waste a drop!
The reason why is because Moen uses a patented "Cup Washer" in the gate of it's diverter spouts. This cup washer mounts "backwards" in the gate so that when you lift the gate, water enters a hole in the "back" of the cup washer causing it to "inflate" with water, and that causes the bottom of the cup washer to bulge outward, sealing off the hole in the diverter spout through which the water flows. It's a surprisingly simple but effective way of completely shutting off the water flow. And, unlike your typical diverter spout that leaks more when the water pressure is higher, the Moen design is opposite. The higher the water pressure, the more firmly that cup washer inflates with water and the more securely it shuts off water flow out the spout. The patent prevents other companies from using this design.
And, the real kick in the ass with other diverter spouts is that if you install a "water saver" shower head, which restricts flow out of the shower head, you simply waste more water because restricting the flow out the shower head simply increases the flow out the spout. Because of the way the 3931 works to shut off the water, water saver shower heads work as intended with the 3931.
(I own a small apartment block and I used to have tenants replacing shower heads because they couldn't get decent shower performance with my old shower heads. That was partly because so much of the water was being wasted out the spout, but also it was because the "water saver" shower heads I had were simply conventional shower heads with flow restrictors in them. That doesn't work because if you restrict the flow of water out of big holes, the water doesn't flow as quickly out of those holes and the result is that the shower head spills water onto you instead of spraying it onto you. To get good shower performance, you need BOTH the smaller holes of a properly designed low flow shower head (I use the (award winning) Niagara N2130 shower head) and a diverter spout that doesn't waste water (the Moen 3931 and others) so that ALL the water flows out the shower head.
3. You can replace the gate, cup washer and diverter rod on the 3931. Just phone up Moen's 1-800 number and ask them to send you a 10644 diverter spout repair kit free of charge (cuz the cup washer on your 3931 is leaking
Enter the part number 10644 into the Search box in the top right corner of the screen, or just go to this web page:
So, you're fretting over the wrong problem.
The issue isn't whether you've got a slip-on or screw-on cuz both kinds of attachments work fine.
The real problem is that most of the diverter spouts of both kinds are shower water wasters. The issue you should be concerning yourself with is "WHICH slip-on should I be using?" cuz, believe it or not, there are great ones and lousy ones.
And, not all water saver shower heads work equally well either.