What is the benefit of low efficiency speakers?

Yes I agree, I can also think of none. That is unless you like Set low watt tube amps. My friend has a 300B Set amp with $6K high sensitive speakers, the Sonatas I believe, rated at 93 db, they are great on some things, jazz, vocals, but they do "bark" on male vocals at a certain fq, it is a 3 way, has a midrange, which I never in 30 yrs listening like the sound of a midrange driver. NEVER.
I have the Seas Thor's a MTM, W18+T25+W18, rated 4ohms, 86db!!, but very very nice in all areas. Take a look at Tyler's MTM, same as mine, I have the kit Thor from Madisound. I'm driving them with the little Jadis Orch Refer, has 4 KT90's. I use it with the tone controls up 98% for the extra dynamics in the bass and especially the highs.
So yes, a 87db speaker does reqire some good wattage to bring forth the dynamics. But to answer your question, NO it does not matter whatsoever, low sensitivity, high sensitivity. What matters MOST is how does the drivers voice the music. Get it. How does the music come forth from the speaker. Natural? No bark in vocals? Soundstage? Does it sound lifelike? Does the sound have fatigue? (all the above is the same question posed in various ways)
My Thors have zero fatigue. The Danish have been working on the Seas drivers for 50 years. I guess they got it right with the Excel line.

BTW, my friend is selling his Silverline and Cayin 300B.
I'd never sell my combo.

Some low efficiency loudspeakers sound very very good. Chario academy 1's are very inefficient, about 82 or 84 db 1 watt/1 M, 4 ohms. Some very efficient speakers sound rather, ah, bad in some ways; JBL L-200's at around 96 or more db/1w/1m. I own a pair of both. If you want efficiency, it usually means very very large units, not easy to deal with in small houses or apts, not easy to move around.

I think Xiekitchen is finally beginning to touch on the substance of the issue.

I believe that the vast majority of speaker designers don't WANT their speakers to have low sensitivity, they choose the drivers for OTHER characteristics. Remember when 'air-suspension' speakers were introduced (in the '50s? by Acoustic Research?)? The goal was good bass response from small enclosures. One way they did that was too decrease the stiffness of the cone's double suspension so that cone travel could increase. Increased travel required more space between the voicecoil and the magnet structure. Increasing the gap reduced sensitivity. So for better bass from a smaller box, we get lower sensitivity. The other drivers (MR, tweeter) have to be of approximately the same sensitivity, so now we have speakers with typical sensitivities of less than 90dB/2.83v./1M instead of 95 - 105dB sensitivity.

BTW, there's a difference between the terms sensitivity and efficiency, but for this discussion, the terms are interchangeable.

1. Low efficiency is a trade-off towards the full range and power handling.