Ye olde booster 2 mechanic. For those unaware, I refer to the cards who's skills read:
Auto: When this Unit intercepts, and you have a <<Clan Name>> Vanguard, during that battle, this Unit's Shield gains +5000.
Soon after its introduction, the skill was thrown on units from most clans, almost to the point of becoming part of the core 'staples' Bushiroad distributes universally. It's reasonable enough in theory; you can save a card or two in your hand by way of this skill (or the threat of it). In a more practical setting, however, the units sporting the skill often fall short and are excluded from the majority of decks.
8,000 power is below average on a Grade 2 unit. Defensively, this is hit by the strongest Grade 1s, most Grade 2s, and all Grade 3s - without a boost. The Unit becomes an easy target. Offensively, this fails to hit the majority Grade 2 and higher Units without a boost. A commitment of an additional card from one's hand becomes necessary to simply hit something, and that's not always possible. In such cases, the Especial Interceptor merely sits on the field, doing half of what a Grade 2 Unit really ought to. This mathematical inferiority combined with the requirement of strictly being a rear-guard makes it unreasonable to play several copies of the card.
Of course, reduced numbers leads to reduced reliability. For as much as the Especial Interceptor may show up on time, it may also appear when your front row is already filled with bigger and better units - a situation where it can't fulfill its purpose. The easiest way to avoid this inconsistency? Bingo! Excluding the Especial Interceptor.
(Decks with selective calling power such as Royal Paladin, Gold Paladin, and Granblue are exempt from this problem and can easily justify their own Especial Interceptors, however.)