But, could you give an example situation where I might come across such a signal ? Maybe then I'll be able to make a judgement - whether or not such a use-cases would ever be one that I'll need to consider. Also, are you implying that such a measurement would have been impossible on the Rigol ?
As mentioned, power supply ripple. Whether it'll be visible on the Rigol or not depends on the intensity of the ripple. I'm sure there are many other things that might qualify, but suspect that most of them will be of interest only to people who are doing analog work.
If
all you're going to do is digital work, and you
know this, then a scope with a relatively noisy frontend isn't going to give you problems, and other things are going to matter a lot more.
Honestly, as a general purpose tool, something like the Rigol is probably going to be just fine. The problem is that you won't necessarily know in advance when you'll need to see signals from your DUT that fall within the noise range of the frontend. Sometimes you're trying to figure out why your circuit is failing and it's only after you see the full characteristics of the signal that you realize what's really going on. If the signal includes swings that fall below the noise floor, you won't see them. Maybe that will matter and maybe it won't, but it may be that you won't realize it until you see it. As a general rule, the quietness of your frontend is like horsepower in a car. Chances are you're not going to need a lot of it but there are some situations in which there just isn't a substitute.
One question I have to ask is: in your region, what are the prices of:
- The Rigol MSO5000 (probably the 5074)
- The Siglent SDS-2104X+
- The Siglent SDS-1104X-E
- The Siglent SDS-1104X-U
- The Instek GDS-1054B
Prices tend to vary a lot across regions, and I think it would be interesting to see what the various options would actually cost you.