This is what happens if you try this strategy in +AI:
1. Enemy will not rush to you, instead they will approach cautiously from different sides, using cover if possible.
2. Enemy will start blind shooting for suppression from different sides, and since you are gathered in one corner, you have no maneuver available to avoid suppression fire, and you have increased suppression from bullets since all mercs are lying close, you start cowering and loosing accuracy.
3. Enemy will shoot GLs and mortars at you, if they have any, and you will have a lot of casualties and suppressed mercs since you are close together and have no room to spread out.
4. Enemy will start slowly approaching, throwing smoke grenades between them and player mercs.
5. Once they are close enough, enemy will start throwing grenades at your wounded and half suppressed mercs.
6. Once they see you cannot resist effectively anymore, they will rush at you in numbers, trying to overwhelm remaining defense.
Also, +AI removes instant vision bonus from scopes and binocs, so the distance between the moment when you see enemy and the moment he can see you and shoot at you is much shorter.
In general, in +AI you want to spread you mercs and be able to control flanks and outflank enemy instead, and any sitting position may become a trap once enemy knows your location and starts shooting blindly for suppression and use grenades/mortar shells and finds cover from you and also attacks you from flanks.
Also, some enemies with low morale will not move out of cover and will wait for you to advance instead, unless their friends have real success in battle, then they can move out and go to help.