[I discuss your 1010101010 data patterns at end of this.]
By the way, harmonics do not exist. They are just an artifact of the sinusoidal basis function used in Fourier Series.
For example: A single spike or impulse or a single fast edge ----- will not display harmonics. Why no harmonics? because some repetition is needed.
However, in a real world circuit with energy storage in the form of inductors and capacitors, you may have a narrow-band frequency response and thus a ringing time response whenever a single fast edge arrives.
You can see this effect, using a series resonant circuit that has moderate dampening (Q of 5 to 50), and a pulse generator with variable pulse width and/or variable frequency. Adjust the generator over a wide frequency range, and you'll find regions where a slight change in frequency or pulse width (duty cycle) causes dramatic change in the stored energy waveform across the inductor or capacitor.
What is happening?
Narrow-band circuitry act as CORRELATORS, having a memory for prior energy input and the phase of the prior energy input.
When your waveform is repetitive, your narrow-band circuit implements the correlation, and remembers phase information.
Regarding your questions
Stored energy causes inter-symbol-interference, and will degrade the data-eye to where there is no square-ness to the data-eye and thus no safe time to sample the voltage for making a decision. Beware of package inductances in high-frequency data transmission; the stored energy is not your bit-error-rate nor data-eye friend.
Some data transmission systems do superimpose multiple waveforms (Intel has tried this, to control the out-of-band energy content in their radios) to create useful peaks and nulls in response of the correlators. This requires very fast circuits.
For 101010 data patterns, which we can describe as "square waves" having exactly 50% duty cycle, only the ODD elements of the sinusoidal basis functions are needed.
As soon as non-50% duty cycles are needed, or you have random data streams, a mix of odd and even basis functions are needed.
Get access to a spectrum-analyzer and a pulse generator set to very weak output (less than 0.1 volt, so as to not damage the spectrum-analyzer). Then explore the spectrum, which is the coefficients of the basis functions, as you vary the RESOLUTION BANDWIDTH.
A wide resolution bandwidth destroys all information about the odd and the even energy contributions.
A narrow resolution bandwidth, which is akin to a precision correlator, lets you examine the ODD and the EVEN contributions.