TLDR; the game experience is just better compared to using full lineups.
I used to play full lineups but now I only play one tank lineup when playing solo. The pros just vastly outweighs cons and makes it a better experience overall. The main pros are(note that I play mostly BRs from 9.0 and up as I am not interested in low to mid tier stuff):
I do not have to deal with CAS - I am not interested in CAS nor fighting it. Whole combined aspect is just not fun for me. Unfortunately the TO mode does not exist and the closest thing to it is just an early stage of the game.
I do not have to deal with spawn campers - current spawn 1-2 spawn points is just a recipe for disaster. Especially combined with crappy map design where you can see spawn from long ranges.
I do not have to deal with bad maps for too long - with each update gaijin makes one ban map on premium accounts less and less valuable. Addition of winter variants of old maps made map ban almost useless for me. Also I do not like big maps as game play is stale most of the time and Battle Mode is just pure pain. There are just not many options what you can do about maps so taking one vehicle can eliminate it. I play more carefully on maps I like and yolo rush on maps I do not care about.
I do not have to deal with crew skills - the differences between bad crew and good crew are too significant so having just one slot where all points go will make it a good crew slot much faster. It also makes experting crew much safer as you do not have to worry about BR changes and breaking lineups.
Stock grind is much faster - when you unlock new vehicle grinding modules with a full lineup is slow as you will not play with stock vehicles most of the time. A lot of vehicles have huge differences in performance while in stock and while at least half upgraded.
I can play vehicles I just want to play - I am personally not interested in all vehicles in tech trees. I have specific needs from vehicles in case of performance and I do not want to spend any time trying to stock grind tanks I do not care about. I am also not interested in stock grinding copy paste vehicles.
Meanwhile cons are just:
Lack of good crew slots while in squad - technically issue but not really as I most of time just take spaa and sit and wait until my squad mates lose all their vehicles.
Lack of spawns on good maps - not much of an issue when there are just few maps that I truly like
Different points carry different weight. Most of the weight(like 80%) is carried by having to deal with CAS as this one is the biggest frustration element in the game for me. TO mode would probably make me switch back to full lineup but seeing that TO thread was on forum for such a long time I do not have any hope I will see it in game. The only thing you get from that thread is that you have to adapt or move to a different game(like there is any). So I took it into heart and adapted just not like those people thought I would. CAS is part of the game and so does the ODL so now it is in your hands to adapt.
I personally never cared about ODL as I do not care if I win or lose. It is not worth caring about it in games built around grind and not quality of gameplay. You can see this in top tier Air RB where gameplay would greatly benefit from decreasing number of players in match as skill would be more rewarded but Gaijin don’t do that because it would demotivate players that are not willing to improve, from grinding more than one tech tree.