Yep, Forum's bonus is pretty worthless and Merc's not going to help a whole lot except in the rare circumstance that you actually use Merc - very situational. Factored in is that around the time I'm making a GP push, I like don't have many markets up anyway, rather going with Caste. Honestly, I don't get excited about Rome as a civ - I think they are rather overrated.
Mall is pretty late and by that standard pretty worthless in a normal game. However, I think Firaxis tailored the game for starts in different eras. When you think about it, Americans and Toku are pretty damn strong with Ind/Modern starts and the Mall would rock. Granted, not many I know play many late starts, but it can be a nice change of pace.
IMO opinion, good UBs provide a tangible benefit regardless of the situation - Extra happies or health, economic boost or lowered maintenance. Salon doesn't really add any value unless going for a culture win and in normal games you should be well on your way to victory before ever getting Astro, if you ever do.