SMA TOLD TO TAXI INTO POS AND HOLD. CTLR CLRED AN ACR TO LAND ON THE RWY AND FORGOT ABOUT THE SMA. SMA ADVISED THE TWR BEFORE AN INCIDENT OCCURRED.

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I WAS WAITING FOR IFR DEP. WAS TOLD TO TAXI INTO POS AND HOLD RWY 28R. RIGHT AFTER THIS ACR X CHKED ON AND RPTED THAT HE WAS ON THE VISUAL FOR THE R. AFTER MORE THAN 1 MIN OF WAITING I TURNED MY ACFT (SMA Y) APPROX 40 DEG TO THE L. THIS ALLOWED ME BOTH TO SEE THE ARRIVING ACFT AND ALSO POS THE ACFT TOWARDS THE TURNOFF. AFTER I DID THIS I NOTICED ACR X WAS GETTING VERY CLOSE. I CONTACTED TWR; SAYING THAT I WAS STILL IN POS AND HOLD ON THE R. TWR CAME BACK WITH A VERY REAL SENSE OF URGENCY AND TOLD ME TO TURN L AND EXIT THE RWY IMMEDIATELY. FORTUNATELY I WAS ALREADY FACING TOWARDS THAT DIRECTION AND QUICKLY GOT OFF THE RWY. I ESTIMATE THAT LESS THAN 1 1/2 SECONDS PASSED BTWN ME XING THE HOLD LINE EXITING THE RWY AND ACR X XING THE THRESHOLD. SHADES OF LAX CROSSED MY MIND. AFTER THIS I WAS AGAIN TOLD TO TAXI INTO POS AND HOLD; AND DEP WAS UNEVENTFUL. I FEEL THAT POS AND HOLD GAINS CTLRS VERY LITTLE WITH SMALL PLANES AND THAT IT SHOULD NEVER BE USED UNLESS DEP IS ENSURED IN A VERY TIMELY FASHION (LESS THAN 30 SECS). I HATE TO THINK WHAT WOULD HAVE HAPPENED IF I HAD NOT BEEN LISTENING TO THE RADIO AND NOTICED THE OTHER PLANE COMING IN. I MIGHT ADD THAT THE TWR FREQ WAS NOT VERY BUSY AT THE TIME. I THINK THE CTLR JUST FORGOT ABOUT ME.