Just my $0.02, but I would definitely add it.
1)A house with one bathroom is only one problem away from an emergency (read: potentially very expensive) plumbing repair bill. If you have two, they can always use the other one until you fix the first one the next day at normal rates. If they only have one, their emergency will be yours until you fix it.
2) Adding a bathroom isn't insurmountable, but it can be very involved. If the house is on a crawlspace, it's much easier because you can get up under the house for the plumbing. If it's on a slab, you'll need to concrete saw and jackhammer out a trench in the floor for drainlines and then reconcrete and retile. It's not impossible, but it can be very expensive. It's not something I'd recommend you try on your own unless you already knew and understood everything I just said in this paragraph.
3) The house is going to be worth more when you go to sell it. Always have an exit in mind, even if you're doing buy and hold.
Good luck!