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The reason for the great count of radio towers near Mecca may be that each company often built its own tower or even for each program its own tower. In most parts of Europe it is common that one tower must be used for all programmes.
However, there are sites with multiple masts. Most of them are or were used also for shortwave, mediumwave and longwave transmission. Towers and masts for this kind of transmission are often the antenna itsself and have high voltage against ground when transmission takes place.
Installing antennas for other radio services on this objects is possible, but requires a special device in form of resonant circuits, in whose coils a cable runs, to bring the signals from ground to high voltage potential.
Such device is expensive and maintenance on the mast requires either switching off the LW, MW or SW-transmitter or access via an insulated platform.
So it is often prefered in such cases to build two towers: one for longer waves and one for UHF and VHF.
Directional antennas for longwave and mediumwave require nearly always two or more masts. For shortwave dipoles spun between two or more towers are used.

Hoever all of the towers on the hills around Mecca seem to be used exclusively for UHF-/VHF-transmission. Abraj Al-Bait can perform the job of several of them perhaps.