IMHO, the extra work and money in putting three bulkheads in a keg is worth it no matter what the usage is. If you have to plug two of them initially, it's not a big deal. What if you want to sell it later and the buyer wants it for a mash tun or HLT?
Even so, I love the fact that I have a thermo and sight in my boil kettle. I got the triplet of thermos at a discount so it wasn't a big cost increase. I know when I'm "about" to boil. The sight easily shows me what my first runnings, total preboil, and postboil volume is. I'd never go back to guessing based on the keg ridges.
A sight on the MLT? What if you heat your strike water in there? I had already decided on the triple bulkheads on the HLT and HLT, why not make them all the same?
Just my opinion of course, you'll get a bunch of different ones here and that's awesome. I'd find out the cost difference from your welder. If it's a few bucks more to put multiple couplings in, go for it.