I'm not entirely sure how XP is calculated at the moment. But, it seems that XP for objectives is worth more than XP for kills. I like this as it encourages people to focus on the objective rather than their personal K:D.
However, you only seem to earn objective bonuses if you are ON the objective when it is captured/defended.
If I'm at an objective and see many teammates are already parked inside cap'ing. I tend to circle outside and try to flank enemies and/or mark their approach path. I'd argue this is a valid way of supporting the capture/defence of the objective. But, as I'm not in the building at the time, I usually find I'm not credited with any objective bonuses.
I'd suggest that if you kill, hit or even suppress any enemy player/bot that is within a certain RADIUS of the objective as it is being captured/defended - even if YOU are not within the capture area at the time - you also be awarded XP for supporting the objective.
This would mean that if you aren't inside the objective zone, you'd only get XP if you actually engaged a nearby enemy. Simply camping behind a rock so you don't die while your teammates do all the work wouldn't net you anything.
So, as mentioned above, flanking or snipers providing overwatch for example would be considered support of the objective.
I just think that actually being inside the objective zone (while of course necessary for the capture) is to narrow view of "playing the objective".