A common stereotype about T20 is that frontline bowlers are not relevant here. It's a batsman's game, and it's all about hitting, and bowlers are just there to get hit.Spoken by Srinath on Extra Cover, Neo Cricket
If there are more T20 and no more ODIs, the specialist bowler is gone
But there is, however, a big advantage that a frontline bowler can provide. A specialist bowler has a clue of how to bowl to most batsmen, in most conditions. Even in this format, a specialist bowler can do significantly better than a bits-and-pieces bowler.
Take the case of India, for instance. They have had their main bowlers getting wickets at good rates, and even as T20 economy rates are going down, they've had decent economies. Their part-timers, however, get smashed all around for more in a single over than the specialists in all four. Likewise, Scott Styris, a stock bowler for NZ, has struggled in T20. If it's a nightmare for a bowler, imagine how bad it may be for a part-time, filler bowler.
On the other hand, some tight overs by a frontline bowler can set back a batting side significantly. A series of tight overs can win a match. Ultimately, even here, bowlers can change a match, so they're not out of it at all.