I myself would only dare camp with the Rainmaker in my hands if my team has the point lead within the last 30 seconds or so, and we've Splatted the entire opposing team before attempting it. Actually Splatting the entire team becomes more difficult post-Bubble-Burst, the longer the survivors remain, especially if said survivors are running Tenacity for the increased Meter gain for their Specials whenever their teammates have been Splatted. Said difficulty continues rising if the survivors remain long enough for their fallen comrades to return to the front lines with them. If the entire enemy team is splatted, it's more likely that my whole team will be making a beeline rush towards the goal space, which is where I'd usually be hustling towards anyways, especially if we don't have the point lead.
Since viewing the in-game instructions and testing out what would happen if the Rainmaker's timer were to reach 0 before even stepping foot online, I've respected the timer put on the Rainmaker, because upon impact, anyone on your team that's around you will get Splatted, especially the Rainmaker holder, and the explosion radius is quite large too. The biggest casualty that can come from the Rainmaker timer reaching 0 is if your entire team's close enough to you to be within that explosion radius and there's more than 10-ish seconds left in the match. I'd mention having the point lead, but I'm certain whole teams would only try to camp with the Rainmaker in their hands if the Rainmaker holding team had the point lead. As the in-game instructions and TheMH have said, the speed of the timer drastically quickens the closer your Rainmaker holder is to your own base. I've not done the testing on how fast the timer goes down, but I'm not willing to take that risk online just yet, since my teammates will likely count on me to hustle over to the goal space as quickly and bravely as possible, and I'll use Squid-powered short hop slight charge shots to make paths for myself if none of my allies are nearby to help achieve that goal.
In reality, regarding the timer, after several (over 20+) matches in the mode, the odds of getting Splatted by the opposing team before the full minute is up is practically guaranteed to happen over 50% of the time, often before even reaching the 30th second, especially if you're like me and use the allotted minute to hustle over to the goal space as quickly and bravely as possible, with the intent of scoring the point lead for your team to withstand enemy advances. While I would love to just go from center stage all the way to the goal space in 1 fell swoop, I wouldn't consider that possibility to be too likely to happen, so I'll settle for and even take solace in having any point lead over the other team if possible.
I'm also not sure how much of your overall movement speed is reduced by holding the Rainmaker, but I'd imagine that running Run & Swim Speed Ups would help counteract the movement penalties incurred by holding the Rainmaker, as would Ink Resistance Up if you're wanting to use Squid-powered short hop slight charge shots atop enemy Ink when you're charging ahead by yourself.
Thank you Hitzel for informing us about the power of camping in this mode and a counter-strategy to it. I myself wasn't even aware that such a tactic would even be viable in this mode, so next time, I'll definitely be on the lookout for such a strat. However, in the "With Anyone" type of setting, without voice chat, I'm not sure how we can inform our allies of this strategy, for their most basic instinct is to rush down the Rainmaker holder and attempt to steal it for their team. We've only got "Booyah!" and "C'mon!" as our D-Pad Sonor commands, and I don't think they're descriptive enough for folks to get our desired message. In the Private and Squad Rooms, there's no such concern there, due to the enabling of voice chat with outside tools like microphones, smartphones, and Skype. Of course, in locals that will likely have separate Wii Us nearby to your teammates, you can just straight up mention this (counter-)strategy to your in-person teammates, so long as the opposing team doesn't hear about it and take action for it.
AnchorTea definitely hit the nail right on the head regarding the role of the Rainmaker holder. He pretty much described my key focus as the Rainmaker holder, namely HUSTLING over to the goal ASAP, using approach route mix-ups between the multiple available routes (if available) and speedrunning methods like Squid-powered short hop slight charge shots whenever possible to maximize the point lead for my team.
EDIT: I also want to support AnchorTea's point about not focusing on Splatting people with the Rainmaker, because if you're like me and you're hustling towards the goal space with a technique like Squid-powered short hop slight charge shots, said slight charge shots will NOT Splat folks in 1 go. The power of those slight charge shots is quite low, and I think it takes 3 of those shots to Splat the majority of folks (said number has not been confirmed yet, by the way), which is far too slow for it to be a safely viable method of Splatting folks. Plus, any time spent trying to Splat folks is time NOT spent hustling towards the goal.