For weapon research this is without a doubt true (those are all so expensive and climb in such little increments, the only one I look to with some regularity are the TEC missile ones because they're decent, and a stepping stone to the Novalith), but I wouldn't shrug off Shield and Hull research since it can impact the early game. I would always be striving to use my full fleet cap first (the 20 ships you mentioned generally would do it) but looking toward researching Hull and Shields early can sometimes yield benefits.
These fields of research just don't add to the maximum hit points, but also add to the regeneration rate of all of your ship's in your fleet. This can be a noteworthy factor early in a game where the fighting is fairly low intensity (especially with militias), and ships have time to regenerate during and immediately after a battle (even running in and out of one).
For Advent, you can do both levels of Basic Shield Projection for a grand total of 900 credits and 75 Crystal which results in 10% more shielding and 10% faster shield regeneration. They also pay the same amount (900 credits and 75 Crystal) to get 10% stronger hulls and 10% faster hull regeneration, but since Advent has lower armor levels the worth of this can be contestable (although with Phase Missiles this can start to matter). But for no Metal (which Disciples often eat early game) and for less than cost of four Disciples you increase survivability of your entire fleet, and the basic shielding is necessary to climb to the ever-useful Guardian.
TEC can pay the same amount (again, 900 credits, 75 Crystal) and get 13% more hitpoints and 5% faster regeneration, and can also sink those same resources into research allowing them to have an increase of 1.5 Armor to all of the ships in their fleet. Not as much regeneration as Advent isn't so great, but getting an Armor increase isn't half bad, and if on the defensive with Repair Bays this can look kind of attractive.
Vasari only has the ability to pay for more Hull and Regeneration at T1, resulting in 13% more hull and 5% faster regeneration, again at the same cost of 900 Credits and 75 Crystal. They seem to get the raw end of the deal here, although they do have stronger ships as a whole so more innate survivability, I guess, and this research is needed for their Reintegration self-repair ability.
With all that said, I agree with you and definitely would not forgo fully utilizing my initial fleet cap in favor of doing this research, but often once I max it out I consider these research fields before upgrading making my first fleet capacity upgrade. Whenever you're not fully utilizing your fleet cap you're at a disadvantage, afterall.
And I'm much more weary of the research beyond T1 that sets you back significantly more Credits, Crystal, and also costs you Metal and generally climbs at about the same increments. But those first upgrades are oh-so-cheap compared to the later ones.