The big advantage of an equatorial mounting for variable star observing is its automatic alignment of the chart showing the variable star and comparison stars. The Dobsonian offers no clue to how to orient the chart, and it can be amazingly difficult to find that alignment if you have no idea what direction in the field is east-west. All you need to do with an equatorial mounting to find this alignment is to rotate the telescope slightly about its polar axis. Of course, it helps to have the polar axis aligned with the earth axis! But aligning it with the north star is sufficient for that purpose with a portable equatorial.