Most of the time guesting for chains is done by friends that do it (mostly) for free, and usually its people that buddy with the leader so they'll also be working on behind the scenes stuff (not always) and leadership shit (building morale, running comps, helping build the armoury, providing an extra body to deter threats yadda yadda)
Sometimes it's someone who needs a break from active warring fac life and will chill somewhere where they can take it easy (again, usually with someone they already know/work with)
Most of the time it doesn't make sense to Merc for rando chains as they're not self-sufficient (no immediate monetary rewards for chains) - esp if you're going to a fac with limited or no tol to make a profit the leader would have to pay your rehab bill+ w/e e supplements he wants you using (boosters mostly, xans are relatively cheap, though that would sweeten the pot), then usually pay at least 1m/hit on top of that, otherwise it would be better for the Merc to guest in a plat/Dia RW fac, as they can make at least that there lol
All that to say, it would be better to guest in a warring fac for rws weekly, esp if you can bounce between plat (or Dia) FACS that can give you decent pays weekly (you won't be making more than members in most cases, but that should be more than enough), you'd still be dealing with recruit and lack of tol often though, so take that into account when looking!
Tl;Dr guesting for wars is better than chains monetarily, look to FR forum to see who's willing to letcha help them, and remember to Jo n before noon on Tuesday, should you be moving around to different wars per week