#3 Encourages financial recklessness
All clubs ought to keep their accounts books well balanced to stand themselves in good stead. In trying to do this, clubs, especially those without global brands should buy players within their means to avoid spending more than they earn. It is well known that English players are very expensive to sign and can only be afforded by the big guns. It is for this reason that Leicester's Harry Maguire can only be signed by a club Manchester United's stature.
This means for a small Premier League side to get themselves a good player that would make them competitive, they have to look beyond the English shores, or pay expensively. Fulham recently signed Jean Michael Seri from Nice, West Ham got Felipe Anderson, Andriy Yarmolenko and Issa Diop, spending the kind of fees they would probably not have wished to spend.
But because there's a fear that the window could shut without signing players that would make them achieve their goals, they end up punching above their weight. If their transfer gambles don't work out, such exorbitant fees could come back to haunt them.
But if the window remained normal, there would be many excellent players that such clubs could get at a relatively cheap cost. Sadly, by the time they will become available, the window will have been slammed shut on them.