It all depends on what you are planning to use it for.
Most of the analog scopes were only 2 channel, and these were perfectly sufficient for both design and repair of most of the analog TVs and radios of the era.
I think that for most purely analog design work 2 channels should be sufficient. For repair work, where you only need to check the presence and 'quality' of a waveform, 2 channels should be sufficient.
Things begin to change if you are designing mixed signal, control, data acquisition, or micro interfaces where you frequently need to monitor the relationship between more than 2 channels. Digital channels on a MSO or a logic analyzer are great for digital debugging, but they tell you almost nothing about the shape of the waveform, and can't measure skew, slew rates, etc.
Even a simple measurement like the power supply ramp-up and reset needs a 4 channel scope, because ramp-up is a concern if there are generally two or more power rails.
For mixed signal and micro controller work, the question to ask would be:
A 4 channel DSO or a 2 channel MSO with 8 or 16 digital channels?
In my case, I'm fortunate enough to have the Agilent MSOX3024A which is a great pleasure to use. Only thing I wish is that I had at least one channel with higher bandwidth such that I could view the integrity of my clock and other critical signals, but the need for that would be occasional, where everything else is frequently used.