We use this mode (ie A/T in ARM) all the time.
If the A/T is engaged (ie switch is up) and speed is "deselected" you will have "ARM" showing in the top left of your EADI.
This means that you have low speed alpha protection (ie it will drop into "MCP Speed" mode if it deems the speed falls too low for the given weight). This can cause occasional problems as it errs of the VERY cautious side and so will kick in and advance the power when you least expect/want it.
It also given you high speed protection (ie will reduce power if you reach Vmo/Mmo). This mode should however never be seen in normal operations.
During the approach with the A/T in ARM if you press the TOGA button, TOGA will annunciate on the EADI and the thrust leavers will advance to the GA power setting.
All in all, it benefits leaving it in the Speed deselected mode (A/T armed for the appoach) in all but gusty conditions when it may have trouble coping, and keeping out of your way!
Hope this helps!