Many people feel like the time, effort, and/or cost of those undergarments may be too much for an individual (or moreso family!) just entering living history. Believe me, taking on a pair of stays for your second sewing project (assuming your first was your shift) can totally be intimidating and in some cases unreasonable. However, like shoes, the lack of proper underwear can detract from the overall effectiveness of your interpretation.
Which is why, as much as we may think we "don't need undergarments right now", au contraire mes amis; A set of appropriate, well made foundation garments should be a priority for the newcomer and veteran alike. If our bodies look like 21st c. bodies in "old clothes" people will key in on that right away. It not only makes our clothing look more like costumes but makes it harder to "buy in" to the idea that we are people from the past.
With the proper undergarments, even less than perfect clothing can be made to look better, because it allows for the proper silhouette. However, even the most accurate of creations will seem ill at ease without that ever significant foundation.
Notice the two originals below (both taken from auction sites and displayed on mannequins that are not in the correct form for the gowns.)
Compare these to an original from the Met displayed on a mannequin designed with a proper foundation and a portrait of Mrs. Daniel Sargent by John Singleton Copley.
The gowns on the proper form look nicer than the ones that are not. I grant you the difference is much more noticeable via the front than the back, but in both instances the curvature of the body is misplaced, making the gowns on the incorrect forms appear lumpy-and in all the wrong places (because, you know, there's a time and a place for lumps- just not where they appear in these two examples). Note specifically from the back view the area near the underarm and from the front view the level where the fullness of the bosom resides.
In latter cases, the foundation provides a smooth line on which the gown can appear to advantage, rather than in the former, in which the gowns appear somewhat frumpy and less than beautiful. Now, add in poor posture (and all of our extra bits) and this effect is even more pronounced on the human body.
So, once you decide to take the plunge to vamp up your kit- I encourage you to never underestimate the importance of your underwear.