Don't sweat it. Three quarters of a pound of weight for a necessary item is part of long distance camping. They are not part of an SUL kit, but they work well enough as part of a UL one. For beyond a few days, you cannot rely on the weatherman. Even in July, the weather in some places is unpredictable enough that I really want the long johns. This is not a "just in case" item, since it will preserve the overall loft of your bag for up to three or four weeks. And, if you are relying on taking a 40F bag down to 30F, you need them, besides being a comfort item for mornings.
If you are out for a week or less, just ignore it...not that big of a deal. If you are out for a couple weeks, you might find your bag is lofting less, usually meaning it is less warm. You need to get comfortable with washing it.
1) Use an agitatorless washer. Make sure there is NO traces of detergent that can ruin good down. Usually, carefull selection of , a washer if you go to a laundromat is needed. Maybe you need a different laundromat. A full wash cycle before doing your bag helps. Unzip the bag.
2) Use down wash. NEVER a detergent. Use about 1/2-1/4 as much as recommended. WATER washes, soaps loosen dirts and oils (not true but you can think of it that way.) Use extra water (large load), gentle cycle, and extra rinse. Run it through, then run it through again without any soap. Hot water is OK. Cold water is OK.
2a) If you want a DWR on the bag, do it now.
3) Like a baby, you bag is very delicate when wet. Fold it, slip your hands under it to carry it to the dryer.
4) Heat does not bother down (up to steam heat anyway) but it *can* damage the shell. A few dryer balls or tennis balls works well to break up clumps. Start drying it and monitor the heat by opening it every couple minutes to check. Commercial and home dryers vary in temp settings.
5) After about 2 hours, it should be fairly dry. You can take it home and hang it for a few days to finish drying.
I do it all at home (no agitator in the washer, extra large dryer) so it is far less painfull than heading out to a laundromat. The bag should loft better than what it was when new. The bag will last about 20 years like that. I put mine on a schedule of about every 14-21 days of use. I don't wory about stains and stuff. Just the overall function. Down bags can take compression for up to a couple days without damage, but you have to fluff them quite a bit after two days. Spend about a minute shaking the bag out to get it to loft again. After a day of hiking, I set my shelter, pad and bag first. Then I shake it out and open it. This is good for about three weeks on the trail before I can notice a loss of loft, it just won't fluff as easy.
I carry the long johns (UL smartwool, light smartwool, heavy smartwool) for keeping the bag claner, for doing laundry on longer trips, and for "oh sh*t" weather since it is beyond what can be predicted. Sometimes I have needed to wear it till 1300 or so. For short weekend trips, I don't always bring it. A set of UL long johns weighs about 3/4 of a pound, as you know.
For shorter SUL trips, don't worry about the bag, they are relativly easy to wash once you know how. But, you will usually be within weather prediction range, so, as always, bring only what is essential given the conditions.
Not knowing conditions, plan for the worst, though. This is the biggest reason that through hikers or long distance hikers do not use a SUL kit. At times, they will need a bit extra of clothing. Most plan on carrying around 30pounds pack weight and plan on two weeks with no resupply. Long johns become part of "necessary items" under those conditions. Even if the base weight is around 6-7 pounds, it better be able to handle the worst the trail can throw at them.